IMA: fix measuring asymmetric keys Kconfig

As a result of the asymmetric public keys subtype Kconfig option being
defined as tristate, with the existing IMA Makefile, ima_asymmetric_keys.c
could be built as a kernel module.  To prevent this from happening, this
patch defines and uses an intermediate Kconfig boolean option named
IMA_MEASURE_ASYMMETRIC_KEYS.

Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Suggested-by: James.Bottomley <James.Bottomley@HansenPartnership.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reported-by: kbuild test robot <lkp@intel.com> # ima_asymmetric_keys.c
is built as a kernel module.
Fixes: 88e70da1 ("IMA: Define an IMA hook to measure keys")
Fixes: cb1aa382

 ("KEYS: Call the IMA hook to measure keys")
[zohar@linux.ibm.com: updated patch description]
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>