fs_config: drop fs_config_files/dirs PRODUCT_PACKAGES requirement

When configuring fs_config_files or fs_config_dirs for file_system
capabilities, drop the requirement that OEMs must add the target
to PRODUCT_PACKAGES. This limits the configuration requirement
to only needing to set the new and preferred TARGET_FS_CONFIG_GEN
or the older TARGET_ANDROID_FILESYSTEM_CONFIG_H method.

Test: That only setting TARGET_FS_CONFIG_GEN results in passwd and
group in the build image.
Change-Id: I818854fa1b3e94edaff59a32bd7cf23cf9b504aa
Signed-off-by: William Roberts <william.c.roberts@intel.com>