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)

Lots of thanks to @vagrantc for helping me with this! :D