rb532: switch to UBI, drop yaffs2 support, use sysupgrade for NAND

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/rb532/base-files/lib/upgrade/platform.sh

@@ -0,0 +1,26 @@
+REQUIRE_IMAGE_METADATA=1
+RAMFS_COPY_BIN=/usr/sbin/nandwrite
+CI_KERNPART=none
+
+platform_check_image() {
+	[ -e /dev/ubi0 ] || {
+		ubiattach -m 1
+		sleep 1
+	}
+	return 0;
+}
+
+platform_pre_upgrade() {
+	nand_do_upgrade "$1"
+}
+
+platform_nand_pre_upgrade() {
+	local board_name="$(cat /tmp/sysinfo/board_name)"
+
+	mtd erase kernel
+	tar xf "$1" sysupgrade-$board_name/kernel -O | nandwrite -o /dev/mtd0 -
+}
+
+platform_do_upgrade() {
+	default_do_upgrade "$ARGV"
+}