ima: fix updating the ima_appraise flag

As IMA policy rules are added, a mask of the type of rule (eg. kernel
modules, firmware, IMA policy) is updated.  Unlike custom IMA policy
rules, which replace the original builtin policy rules and update the
mask, the builtin "secure_boot" policy rules were loaded, but did not
update the mask.

This patch refactors the code to load custom policies, defining a new
function named ima_appraise_flag().  The new function is called either
when loading the builtin "secure_boot" or custom policies.

Fixes: 503ceaef ("ima: define a set of appraisal rules requiring file signatures")
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>