efivars: check for EFI_RUNTIME_SERVICES

The efivars code requires EFI runtime services to function, so check
that they are enabled.

This fixes a crash when booting with the "noefi" kernel parameter, and
also when mixing kernel and firmware "bitness", e.g. 32-bit kernel with
64-bit firmware.

Tested-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>