build.sh: add support for AVB signing

Adds support for signing the boot image when AVB_SIGN_BOOT_IMG is set.
There following flags need to be defined for signing:
- AVB_BOOT_PARTITION_SIZE
- AVB_BOOT_KEY
- AVB_BOOT_ALGORITHM

Bug: 176930600
Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: I7666a12d332bf6866cd85ec43d33424cf46dcac6
1 file changed