[-d root] should override configuration (ROOT_DEV)

schnapps -d /dev/<root partition> works as expected when the node is booted to the default /dev/mmcblk0p1 but it does not when booted to an USB partition (maybe also SSD but could not test since u-boot GPT support is not implemented).

When booted to an USB partition (interior USB 2.0 with MBR) the root path specified with -d is entirely neglected and schnapps only deals with the current root partition. Curiously however, manipulating /etc/schnapps/config with ROOT_DEV="/dev/<root partition>" works as expected when booted to an USB partition.

Tested with HBS | HBD | Master and reproduces reliably on each installation.