localmodconfig: Reset certificate paths

localmodconfig: Reset certificate paths

When using `make localmodconfig` and friends, if the input config comes
from a kernel that was built in a different environment (for example, the
canonical case of using localmodconfig to trim a distribution kernel
config) the key files for module signature checking will not be available
and should be regenerated or omitted. Otherwise, the user will be faced
with annoying errors when trying to build with the generated .config:

make[1]: *** No rule to make target 'keyring.crt', needed by 'certs/x509_certificate_list'.  Stop.
Makefile:1576: recipe for target 'certs/' failed

Link: http://lkml.kernel.org/r/1461696721-3001-1-git-send-email-bpoirier@suse.com
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>