optionally use a canned fs_config when building ext4 fs

optionally use a canned fs_config when building ext4 fs

In order to accurately re-create old images after changes to
android_filesystem_config.h, we need make_ext4fs and mkuserimg.sh to
be able to take a file containing the desired
uid/gid/mode/capabilities for each path in the filesystem, and use
that instead of the fs_config function that is built in to the binary.

The target_files already contains this file, which is created at the
time of the original build by running the binary fs_config for each
path in the build.

Change-Id: I501dc9544de37a592c1e831b6bf785c086cb6912