bmips: add subtargets for each SoC

BMIPS is a generic arch that can be used for multiple Broadcom SoCs, each one
with its own specific drivers, so instead of having a huge kernel supporting
all of them, let's switch to a subtarget per SoC like other OpenWrt targets.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

target/linux/bmips/bcm6368/base-files/etc/uci-defaults/09_fix_crc

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+. /lib/functions.sh
+
+case "$(board_name)" in
+comtrend,vr-3025u)
+	mtd fixtrx firmware
+	;;
+esac
+
+exit 0