base-files: upgrade: add case to export_bootdevice

The factory uboot of the Turris Omnia boots with "root=b301", and we
instruct new users to sysupgrade from there (e.g. method 1, step 7).
Currently, this will fail with "Unable to determine upgrade device".
Add a new case to export_bootdevice, which parses the hex argument.

Ref: https://github.com/openwrt/openwrt/pull/2340#issuecomment-561317688
Fixes: 2e5a0b81ec ("mvebu: sysupgrade: sdcard: keep user added partitons")
Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Klaus Kudielka <klaus.kudielka@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 3a4f587c46)

package/base-files/files/lib/upgrade/common.sh

@@ -103,7 +103,7 @@ get_magic_long() {
 }
 
 export_bootdevice() {
-	local cmdline bootdisk rootpart uuid blockdev uevent line
+	local cmdline bootdisk rootpart uuid blockdev uevent line class
 	local MAJOR MINOR DEVNAME DEVTYPE
 
 	if read cmdline < /proc/cmdline; then
@@ -139,6 +139,18 @@ export_bootdevice() {
 			/dev/*)
 				uevent="/sys/class/block/${rootpart##*/}/../uevent"
 			;;
+			0x[a-f0-9][a-f0-9][a-f0-9] | 0x[a-f0-9][a-f0-9][a-f0-9][a-f0-9] | \
+			[a-f0-9][a-f0-9][a-f0-9] | [a-f0-9][a-f0-9][a-f0-9][a-f0-9])
+				rootpart=0x${rootpart#0x}
+				for class in /sys/class/block/*; do
+					while read line; do
+						export -n "$line"
+					done < "$class/uevent"
+					if [ $((rootpart/256)) = $MAJOR -a $((rootpart%256)) = $MINOR ]; then
+						uevent="$class/../uevent"
+					fi
+				done
+			;;
 		esac
 
 		if [ -e "$uevent" ]; then