Add selinux and verity support to custom image generation.

New custom image configuration variables:
- CUSTOM_IMAGE_SELINUX, set to "true" if the image supports selinux.
- CUSTOM_IMAGE_SUPPORT_VERITY, set to "true" if the product supports verity.
- CUSTOM_IMAGE_VERITY_BLOCK_DEVICE

Also changed the staging directory name to the mount point, like we do
for other images built by the build system.

Bug: 19609718
Change-Id: I6bbf06b79eee63e4c77834f2e6f1d5a7f7e00a12
(cherry picked from commit 7d51a402)