MKBOOTIMG override were missing in two places

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