Make AVB work when BOARD_USES_RECOVERY_AS_BOOT is set to true.

Often BOARD_USES_RECOVERY_AS_BOOT is used together with
BOARD_BUILD_SYSTEM_ROOT_IMAGE, but with this combination we're currently
not adding a hash footer to boot.img. Fix this.

Bug: 31119415
Test: Built with BOARD_USES_RECOVERY_AS_BOOT set to true and inspected images.
Merged-In: If033e5c93f5b71fd17405af7d970dc3f0540a1d2
Change-Id: I2f10bbffb0ba2467d53e3e62d5379c3ab7236063