mntreform-config: set CONFIG_ENV_IS_NOWHERE=y

See merge request !9

This is to avoid that u-boot reads the environment from emmc. This is
another step to make sure that accidentally running "saveenv" doesn't
mess anything up. It also makes sure that a u-boot image flashed to the
SD-Card really provides a pristine environment without external
influence.

use distro_bootcmd

See merge request !6

 - this partially reverts b3a8287b which prevented BOOTENV as set by
   config_distro_boot.h being appended to CONFIG_EXTRA_ENV_SETTINGS
 - distro_bootcmd can do everything the nested loop can do:
     * boot from usb
     * arbitrary boot order
     * set arbitrary boot partition
     * search arbitrary list of prefixes
     * load a boot.scr
 - in addition to that, distro_bootcmd:
     * is the standard boot mechanism for distros (see doc/README.distro)
     * thus we can replace hard-to-read custom code
     * works with extlinux.conf (needed for debian-installer)

try booting from USB first

See merge request !5

unset devnum so that the loop variable takes precedence

See merge request !4

use prefix as loop variable so that it can be reused in boot.scr

See merge request !3

boot using boot.scr

See merge request !2