checkfc: add attribute test

Enable checkfc to check *_contexts against a set of valid attributes
which must be associated with all types in the contexts file that
is being checked.

Since it's imperative that checkfc knows which file its checking to
choose the proper attribute set, the -s option is introduced to
indicate the service_contexts file. The property_contexts file continues
to use the existing -p and file_contexts requires no specification, aka
it's the default.

Failure examples:
file_contexts:
Error: type "init" is not of set: "fs_type, dev_type, file_type"

service_contexts:
Error: type "init_exec" is not of set: "service_manager_type"

property_contexts:
Error: type "bluetooth_service" is not of set: "property_type"

Change-Id: I62077e4d0760858a9459e753e14dfd209868080f
Signed-off-by: William Roberts <william.c.roberts@intel.com>