wic: allow multiple /boot partitions with different content

wic: allow multiple /boot partitions with different content

It can be useful to have multiple partitions with '--source bootimg-partition'
but different content.

E.g. for TI AM335x, one boot partition can contain an first stage
bootloader ("MLO"), while the real bootloader and kernel plus devicetree
are in another one.

Patch allows to specify multiple IMAGE_BOOT_FILES with optional "_label-XXX"
or "_uuid-XXX" overrides.

E.g. with this patch, a .wks file with

| part --source bootimg-partition ... --label=mlo --active
| part --source bootimg-partition ... --label=boot0
| part --source bootimg-partition ... --label=boot1

and a recipe with

| IMAGE_BOOT_FILES_label-mlo = "\
|   MLO-${MACHINE}.img;MLO \
| "
|
| IMAGE_BOOT_FILES_label-boot0 = "\
|   u-boot-${MACHINE}.img;u-boot.img \
|   zImage \
| "
|
| IMAGE_BOOT_FILES_label-boot1 = "${IMAGE_BOOT_FILES_label-boot0}"
|
| WICVARS += " \
|   IMAGE_BOOT_FILES_label-mlo \
|   IMAGE_BOOT_FILES_label-boot0 \
|   IMAGE_BOOT_FILES_label-boot1 \
| "

is possible.  It will create one partition with the MLO and two redundant
ones with the uboot + kernel.

Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>