gluon-core: add sh file to get model name etc.

package/gluon-core/files/lib/gluon/functions/model.sh

+. /lib/functions.sh
+
+# This must be generalized as soon as we support other OpenWRT archs
+. /lib/ar71xx.sh
+
+
+ar71xx_board_detect
+
+local board_name="$AR71XX_BOARD_NAME"
+local model="$AR71XX_MODEL"
+
+get_arch() {
+	echo 'ar71xx'
+}
+
+get_board_name() {
+	echo "$board_name"
+}
+
+get_model() {
+	echo "$model"
+}