procd: Add cpu string to board detection

This patch is in relation to
https://github.com/openwrt/openwrt/pull/1307 to help fix architecture
detection in LuCI which was added in
https://github.com/openwrt/luci/commit/b8f32b6da7accc672c5887e894a861de3f806cf5

Signed-off-by: Chris Blake <chrisrblake93@gmail.com>

system.c

@@ -65,6 +65,7 @@ static int system_board(struct ubus_context *ctx, struct ubus_object *obj,
 
 			if (!strcasecmp(key, "system type") ||
 			    !strcasecmp(key, "processor") ||
+			    !strcasecmp(key, "cpu") ||
 			    !strcasecmp(key, "model name"))
 			{
 				strtoul(val + 2, &key, 0);