The barebox,environment binding documentation notes following for
the device-path property's second string:
> <partname> can be the label for MTD partitions, the number for DOS
> partitions (beginning with 0) or the name for GPT partitions.

This doesn't work currently because the named partitions are realized as
symlinks and those aren't searched by device_find_partition.

Fix this by having symlinks feature an appropriate partname if the cdev
they link at has one and then have device_find_partition search those as
well.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>