distro_bootcmd: add NVME support

Some boards support NVME drives. We should be able to use them as boot
devices.

NVME access requires running 'nvme scan'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>