Add support for building vendor_ramdisk-debug.img

Existing BoardConfig.mk variable:
BOARD_BUILD_VENDOR_RAMDISK_IMAGE := true

generates:
out/target/product/<name>/vendor_ramdisk.img

Extend the above to additionally generate:
out/target/product/<name>/vendor_ramdisk-debug.img

as the two ramdisks should be built together.

Test: make
Change-Id: Ief572e30c9225d0f5569e9ff89096c69accafc7f