defconfig: msm: Enable the config SYSTEM_TRUSTED_KEYS

Enable SYSTEM_TRUSTED_KEYS and add certification for x509 signing
for system.

CRs-Fixed: 1116507
Change-Id: I073c1cf8bb1e9bcdee11975e5d072db9c462ee13
Signed-off-by: Venkatesh Yadav Abbarapu <vabbar@codeaurora.org>
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>