rauc: prepare support for locating the config in /usr or /run

RAUC PR #1557[1] extends RAUC to look up its system.conf in /etc/rauc,
/run/rauc or /usr/lib/rauc in that order.

The RAUC recipe understandably hardcodes the assumption that the
system.conf must be supplied and that it must be located in /etc/rauc.

To make the recipe more useful with RAUC once #1557 is merged, let's do
two tiny adjustments:

  - Add RAUC_CONF_DIR, which default to /etc/rauc, but can be set by
    the user to be /usr/lib/rauc instead for hermetic-usr use cases.

  - Add RAUC_CONF_INSTALL, which allows skipping installation of the
    config altogether. This is useful if the RAUC config is always
    dynamically generated in /run/rauc and having an always masked
    /usr/lib/rauc/system.conf would just be confusing.

To avoid breaking existing users, the defaults are chosen, so the
system.conf continues to be installed in /etc. This PR does not break
anything for current RAUC versions, so it should be fine to merge it
independent of the update to a future RAUC release that integrates
the upstream PR.

[1]: rauc/rauc#1557

Signed-off-by: Ahmad Fatoum <[email protected]>

README.rst

@@ -88,9 +88,9 @@ you have to follow at least the following steps:
      DISTRO_FEATURES += "rauc"
 
 2. Add a ``rauc_%.bbappend`` in your device-specific (BSP) layer
-   that installs your RAUC system configuration file under
-   ``/etc/rauc/system.conf``. For information on how to write the RAUC
-   update file, please refer to the RAUC user documentation [1]_::
+   that installs your RAUC system configuration, so RAUC can find it.
+   For information on how to write the RAUC update file, please refer to
+   the RAUC user documentation [1]_::
 
      FILESEXTRAPATHS:prepend := "${THISDIR}/files:"
      SRC_URI:append := " file://system.conf"

recipes-core/rauc/files/system.conf

@@ -1,6 +1,6 @@
-## This is an example RAUC system configuration. This file will be installed
-## into /etc/rauc/system.conf on your target and describes your system from the
-## perspective of the RAUC update service.
+## This is an example RAUC system configuration. This file will be be looked up
+## in {/etc,/run,/usr/lib}/rauc/system.conf on your target and describes your
+## system from the # perspective of the RAUC update service.
 ##
 ## Adapt and extend the below configuration to your needs and place it in the
 ## BSP layer of you project. Create a rauc .bbappend file that adds this file

recipes-core/rauc/rauc-target.inc

@@ -1,10 +1,12 @@
 RAUC_KEYRING_FILE ??= "ca.cert.pem"
 RAUC_KEYRING_URI ??= "file://${RAUC_KEYRING_FILE}"
+RAUC_CONF_INSTALL ??= "1"
+RAUC_CONF_DIR ??= "${sysconfdir}/rauc"
 
 RRECOMMENDS:${PN} = "squashfs-tools"
 
 SRC_URI:append = " \
-  file://system.conf \
+  ${@oe.utils.conditional('RAUC_CONF_INSTALL', '1', 'file://system.conf', '', d)} \
   ${RAUC_KEYRING_URI} \
   file://rauc-mark-good.service \
   file://rauc-mark-good.init \
@@ -22,20 +24,23 @@ inherit systemd update-rc.d
 do_install () {
 	meson_do_install
 
-        # Create rauc config dir
-        # Warn if system configuration was not overwritten
-        if ! grep -q "^[^#]" ${WORKDIR}/system.conf; then
-                bbwarn "Please overwrite example system.conf with a project specific one!"
+        if [ "${RAUC_CONF_INSTALL}" = 1 ]; then
+                # Create rauc config dir
+                # Warn if system configuration was not overwritten
+                if ! grep -q "^[^#]" ${WORKDIR}/system.conf; then
+                        bbwarn "Please overwrite example system.conf with a project specific one!"
+                fi
+
+                install -d ${D}${RAUC_CONF_DIR}
+                install -m 0644 ${WORKDIR}/system.conf ${D}${RAUC_CONF_DIR}/system.conf
         fi
-        install -d ${D}${sysconfdir}/rauc
-        install -m 0644 ${WORKDIR}/system.conf ${D}${sysconfdir}/rauc/system.conf
 
         # Warn if CA file was not overwritten
         if ! grep -q "^[^#]" ${WORKDIR}/${RAUC_KEYRING_FILE}; then
                 bbwarn "Please overwrite example ca.cert.pem with a project specific one, or set the RAUC_KEYRING_FILE variable with your file!"
         fi
-        install -d ${D}${sysconfdir}/rauc/
-        install -m 0644 ${WORKDIR}/${RAUC_KEYRING_FILE} ${D}${sysconfdir}/rauc/
+        install -d ${D}${RAUC_CONF_DIR}/
+        install -m 0644 ${WORKDIR}/${RAUC_KEYRING_FILE} ${D}${RAUC_CONF_DIR}/
 
         install -d ${D}${systemd_unitdir}/system/
         install -m 0644 ${WORKDIR}/rauc-mark-good.service ${D}${systemd_unitdir}/system/
@@ -45,6 +50,8 @@ do_install () {
         install -m 755 "${WORKDIR}/rauc-mark-good.init" "${D}${sysconfdir}/init.d/rauc-mark-good"
 }
 
+FILES:${PN} += "${RAUC_CONF_DIR}"
+
 PACKAGES =+ "${PN}-mark-good"
 
 # some magic to get the tool to interact with bootloader storage