tools: add script to verify non-conditional config fragments

Add a script which can be used to verify that the non-conditional
config fragments are consistent, i.e. none of them disappear when
merged into an an allnoconfig.

Change-Id: I390823ead7ca5540f969a73e641855b8d9f55008
Signed-off-by: Steve Muckle <smuckle@google.com>