octeon: determine sysupgrade-* directory for upgrade automatically

octeon: determine sysupgrade-* directory for upgrade automatically

So far, the sysupgrade-* folder used during upgrade in octeon was
hardcoded to contain the board name. Therefore, changing board name
or BOARD_NAME variable in image/Makefile might have broken upgrade.

Improve this by adding a step to determine the folder name via
a wildcard, as it is done for generic nand_upgrade_tar() in
lib/upgrade/nand.sh.

While this still does not remove the problem for existing devices
(which still have the old script), it will entirely remove the
issue on newly added devices on this platform.

Signed-off-by: Donald Hoskins <grommish@gmail.com>
[split into separate patch, add commit message, add return values]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>