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: aosp_x86_64-eng 'm' and 'm dist' successful.
Change-Id: If033e5c93f5b71fd17405af7d970dc3f0540a1d2