A first stage bootloader can read fat and ext4 filesystems, and
even both can be compiled-in at the same time.
But then xload has a hardcoded fat filesystem mount option which
renders ext4 unusable.
This patch tries to mount it as ext4 if the fat attempt fails.

Then, a typical use case of an ext4 formatted filesystem
is it to be a standard linux filesystem, which contains
boot-related files in /boot. So, when searching for the second stage
bootloader, try /boot/barebox.bin after not finding it in /barebox.bin

Signed-off-by: Vicente Bergas <vicencb@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>