base-files: define yes/no as valid boolean options

ubox validate_data defines yes/no as valid boolean options, do the same in
config_get_bool too.

Signed-off-by: Luka Perkov <luka@openwrt.org>

SVN-Revision: 41405

package/base-files/files/lib/functions.sh

@@ -102,8 +102,8 @@ config_get_bool() {
 	local _tmp
 	config_get _tmp "$2" "$3" "$4"
 	case "$_tmp" in
-		1|on|true|enabled) _tmp=1;;
-		0|off|false|disabled) _tmp=0;;
+		1|on|true|yes|enabled) _tmp=1;;
+		0|off|false|no|disabled) _tmp=0;;
 		*) _tmp="$4";;
 	esac
 	export ${NO_EXPORT:+-n} "$1=$_tmp"