The current code tests if a partition (i.e. disk0.0) exists and instead
of mounting boot from this partition it uses the whole device (disk0).
This only works because the the FAT code accepts a MBR as input and
automatically skips it.

Let the code use the partition to mount /boot instead as it was
intended. We don't have to stat() the partition device, since this
error will be caught by mount() anyway, so remove the unnecessary
stat().

Reported-by: Peter Mamonov <pmamonov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>