add packing/progmem attribute to usb descriptor structs

[bnewbold]

Original author: [email protected] (November 16, 2010 09:36:30)

the descriptors for the virtual com port and the bootloader in
libmaple/core/usb/descriptor.h/c and maple-bootloader/descriptors.h/c can be made into more organized fielded structs as long as the packing decorator is used.

this was tried before without the packing decorator and, due to padding, the descriptors were malformed

additionally generic support for statically locating structs in flash (as opposed to ram) can be made easily and added as a progmem macro as in avr-gcc. this was done for the k1, but the linker mods never made it into the lib maple linkers.

Original issue: http://code.google.com/p/leaflabs/issues/detail?id=32