use distro_bootcmd

 - 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)