fs_config: limit characters for AID_<name> sections

When generating files restrict the characters allowed in
AID_<name> to upercase, numbers, and underscores.

This detects errors ahead of time for generated C files as
well as handles seperation characters for passwd/group files.

This also lends itself to automatic detection of collisions
on friendly names, since freindly names are the lowercase
version of <name>.

Test: That invalid aid values result in a build failure.
Test: That the output files are consistent with ones before
this change (hash and diff checks)
Change-Id: Ie8ec44c1157ba9c22100e9169d9187f615e71280
Signed-off-by: William Roberts <william.c.roberts@intel.com>