Support signing custom images with AVB HASH or AVB HASHTREE

`make custom_images` supports to build different kinds of *non-droid* images,
e.g., odm.img. Adding the support of signing them with either AVB HASH footer
or AVB HASHTREE footer. The user can use HASH for small images and
HASHTREE for large images.

Sample signing configurations:
 * AVB HASH footer:
   - CUSTOM_IMAGE_AVB_HASH_ENABLE := true
   - CUSTOM_IMAGE_AVB_ADD_HASH_FOOTER_ARGS := --append_to_release_string my_odm_image

 * AVB HASHTREE footer:
   - CUSTOM_IMAGE_AVB_HASHTREE_ENABLE := true
   - CUSTOM_IMAGE_AVB_ADD_HASHTREE_FOOTER_ARGS := --fec_num_roots 8

 * Using custom signing key:
   - CUSTOM_IMAGE_AVB_ALGORITHM := SHA256_RSA2048
   - CUSTOM_IMAGE_AVB_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem

Bug: 36701014
Test: `make custom_images` with AVB HASH footer
Test: `make custom_images` with AVB HASHTREE footer
Test: `make droid` to check system.img is still properly signed with AVB HASHTREE
Test: `make droid` to check vendor.img is still properly signed with AVB HASHTREE

Change-Id: I8dc420e12e37e9a631345c0cd883339db05d489f