Add boot_image module type

Adds a boot_image module type. Follow on changes will add instances of
boot_image to represent the ART and framework boot images in the art
and frameworks/base repositories respectively.

For the moment they retrieve their associated boot image configuration
from the map returned by genBootImageConfigs() and leave the actual
work of creating the ninja rules to create the boot images to the
dex_bootjars singleton.

Bug: 177892522
Test: m droid
Change-Id: Ib69701de0d24d996bb8e8be7a20b941be907390e