Add the option for custom_image to be AVB or NONAVB

Check BOARD_AVB_$(call to-upper,$(partition))_KEY_PATH to decide whether custom_image should sign AVB or not. If key path isn't set, the custom image will be excluded from AVB and copied to /IMAGES in target-files directly. This allows vendor to use custom_images flow packing unsigned image.
And to every non-avb custom partition, one image whose name is partition name must be added in its BOARD_<CUSTOM_PARTITION>_IMAGE_LIST.

BOARD_CUSTOMIMAGES_PARTITION_LIST := tvconfig
BOARD_TVCONFIG_IMAGE_LIST := \
    device/xxxx/yyyy/tvconfig.img \
    device/xxxx/yyyy/tvconfig_custom1.img

Test:
1) Build image, target-files, OTA package by m and m dist
2) Sign images by sign_target_files_apk.py

Fix: 285227850
Change-Id: I7477dafe023e4b168f0f08fb7aedd9e511a60e1b