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>
diff --git a/tools/fs_config/README b/tools/fs_config/README
index f4a59ca..b1507a7 100644
--- a/tools/fs_config/README
+++ b/tools/fs_config/README
@@ -82,7 +82,8 @@
 Where:
 
 [AID_<name>]
-  The <name> can be any valid character for a #define identifier in C.
+  The <name> can contain characters in the set uppercase, numbers
+  and underscores.
 
 value:
   A valid C style number string. Hex, octal, binary and decimal are supported.