util/intelmetool: Add additional helpful error messages

Add more verbose error message for common problems on modern
operating systems, like Secure Boot and CONFIG_STRICT_DEVMEM.

Change-Id: Ie3361910d48271bcc2cd3b4b74937fbc5df0a176
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/25401
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

util/intelmetool/intelmetool.c

@@ -491,7 +491,8 @@ int main(int argc, char *argv[])
 	#ifndef __DARWIN__
 		fd_mem = open("/dev/mem", O_RDWR);
 		if (fd_mem < 0) {
-			perror("Can not open /dev/mem");
+			perror("Can not open /dev/mem. Do you have disabled "
+			       "Secure Boot ?");
 			exit(1);
 		}