Pass the file_contexts configuration to mkyaffs2image and make_ext4fs.

This configuration is used to determine the file security contexts
to set on the files in the yaffs2 or ext4 image that is generated.
This is similar to the android_filesystem_config.h tables for determining
the UID/GID/mode, but as a configuration rather than code.

Change-Id: I7c6f59fe7cb8bc4c86c8790f991b12dd93a61f68