Even when u-boot comes from the SD-Card it loads environment variables
from emmc. This means that if somebody accidentally calls "saveenv" with
the wrong environment variables, then there is no easy way to make
u-boot work again and even a freshly flashed SD-Card will fail to work.
If somebody really needs "saveenv" then they can recompile u-boot with
CONFIG_CMD_SAVEENV enabled.

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