fs_config: introduce TARGET_FS_CONFIG_GEN

Introduce TARGET_FS_CONFIG_GEN, an option for generating
android_filesystem_config.h files from multiple intermediate
file formats. See the README for more details.

Change-Id: Ibdb2f65702657802e34b0cec18fb43709d7452bb
Signed-off-by: William Roberts <william.c.roberts@intel.com>