fs_config: introduce group generator

Introduce a generator that outputs group files per man(5) group.

Succinctly, the output is a colon delimited string containing the following
fields:
  * group name
  * encrypted password (optional)
  * gid (int)
  * userlist (str,...)

Multiple colon delimited lines may exist, but will not be separated
across lines.

Sample generator output:
foo::2900:
foo_bar::2901:
custom_oem1::2902:

Test: That make group produces the group file.
Change-Id: Idd3fe925a09a227c6e894e1b5d2b3873b01531c6
Signed-off-by: William Roberts <william.c.roberts@intel.com>