config_distro_bootcmd: Scan all partitions for boot files

Not all devices use the convention that the boot scripts are on the
first partition. For example on chromebooks it seems common for the
first two partitions to be ChromeOS kernel partitions.

So instead of just the first partition scan all partitions on a device
with a filesystem u-boot can recognize.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>