mount-usb switched to dynamic USB device detection a while back,
so eliminate instances of CONFIG_BOOT_USB_DEV, and derive the
mounted USB device from /etc/mtab in the one place where it's
actually needed (usb-scan). Clean up areas around calls to
mount-usb for clarity/readability.
Addresses issue #673
Test: Build Librem 13v4, boot ISO file on USB
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>