AVB: If building recovery.img, add AVB hash footer.

In some non-A/B setups, recovery.img is still being used. If AVB is
enabled, we currently don't add a hash footer to recovery.img nor do
we include the hash digest in vbmeta.img. This CL fixes that.

This was tested on a build with the following settings

 TARGET_NO_RECOVERY := false
 BOARD_USES_RECOVERY_AS_BOOT := false
 BOARD_BUILD_SYSTEM_ROOT_IMAGE := false
 BOARD_RECOVERYIMAGE_PARTITION_SIZE := 33554432
 BOARD_AVB_RECOVERY_ADD_HASH_FOOTER_ARGS := --prop foo:bar

and then it was verified using 'avbtool info_image' that recovery.img
has a hash footer and a 'foo' property with the value 'bar'. This was
also checked successfully for vbmeta.img.

Test: See above.
Bug: None
Change-Id: I98124d5661ea768411416fa8d2a2ae6cc664fdc8