ima: add check for enforced appraise option

The "enforce" string is allowed as an option for ima_appraise= kernel
paramenter per kernel-paramenters.txt and should be considered on the
parameter setup checking as a matter of completeness. Also it allows futher
checking on the options being passed by the user.

Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>