Force /etc to always be RW

We don't want to ever have the user boot into the system with /etc being read only. This leads to unpredictable behavior and should never be allowed to happen.

build-image.sh

@@ -154,7 +154,7 @@ echo "
 LABEL=frzr_root /var       btrfs     defaults,subvolid=256,rw,noatime,nodatacow,nofail                                                                                  0   0
 LABEL=frzr_root /home      btrfs     defaults,subvolid=257,rw,noatime,nodatacow,nofail                                                                                  0   0
 LABEL=frzr_root /frzr_root btrfs     defaults,subvolid=5,rw,noatime,nodatacow,x-initrd                                                                                  0   2
-overlay       /etc       overlay noauto,x-systemd.requires=/frzr_root,lowerdir=/etc,upperdir=/frzr_root/etc,workdir=/frzr_root/.etc,comment=etcoverlay    0   0
+overlay       /etc       overlay noauto,x-systemd.requires=/frzr_root,x-systemd.rw-only,lowerdir=/etc,upperdir=/frzr_root/etc,workdir=/frzr_root/.etc,comment=etcoverlay    0   0
 " > /etc/fstab
 
 echo "