bmips: split base-files into subtargets

Like done for several targets already, splitting base-files into
subtarget will provide smaller images due to more specific
distribution of files per subtarget and allow to use default cases
more effectively.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/bmips/generic/base-files/lib/upgrade/platform.sh

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+PART_NAME=firmware
+REQUIRE_IMAGE_METADATA=1
+
+platform_check_image() {
+	return 0
+}
+
+platform_do_upgrade() {
+	case "$(board_name)" in
+		*)
+			default_do_upgrade "$1"
+			;;
+	esac
+}