GitLab

The change follows the patten found in "Package OTA" and other places
to allow for overriding the mkbootimg command.

Now pass MKBOOTIMG variable to get make_recovery_patch and add_image_to_target
to use the tool set in BOARD_CUSTOM_MKBOOTIMG for boot/recovery image
generation.

Change-Id: I78533c25e87c2750eb24ac1bf39e4b7ca321a441