bootblock: Guard CMOS rewrite in disable/enable RTC

This ensures that there's only one disable/enable cycle for
the entire rewrite instead for every single byte.

Change-Id: Ic06e6dcb08976d158ff784660838c0fbad875176
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/1869
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/arch/x86/include/bootblock_common.h

@@ -27,9 +27,11 @@ static void sanitize_cmos(void)
 		unsigned char *cmos_default = (unsigned char*)walkcbfs("cmos.default");
 		if (cmos_default) {
 			int i;
+			cmos_disable_rtc();
 			for (i = 14; i < 128; i++) {
 				cmos_write(cmos_default[i], i);
 			}
+			cmos_enable_rtc();
 		}
 	}
 }