Using a test key to sign boot-debug.img if needed

When /boot partition is chained in AVB signing, libavb will try to
read AVB footer from the end of /boot partition, even if the device is
unlocked. However, this makes device unbootable when the unsigned
boot-debug.img is flashed on /boot partition. This CL uses a test key
from external/avb to sign the boot-debug.img if /boot partition is
chained.

Bug: 126493225
Bug: 129508966
Test: Enables chain partition for boot.img, `make bootimage_debug` then
      checks `avbtool info_image --image $OUT/boot-debug.img`
Test: `make bootimage_debug-nodeps`
Test: boot a device with a chained boot-debug.img

Change-Id: I870cb70c70b7d4d7a30b77bed58cbca6e007d5e3