base-files: upgrade: add case to export_bootdevice

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: 2e5a0b81ec29 ("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 3a4f587c465a0531b820ca1a3e90eb05c4927b68)