barebox: support auxiliary config build

barebox: support auxiliary config build

Adds support to build barebox with an auxiliary config.

This is useful for building an SPL (Secondary Program Loader) in
addition to the traditional TPL (Tertiary Program Loader). The
Beaglebone Black for example has two barebox configurations:

- am335x_defconfig builds the full barebox bootloader with device
  tree

- am335x_mlo_defconfig builds the smaller MLO bootloader that loads
  the full barebox bootloader from the eMMC or SD card.

Tested with the following defconfig:

  # architecture
  BR2_arm=y
  BR2_cortex_a8=y
  BR2_ARM_EABIHF=y

  # system
  BR2_TARGET_GENERIC_HOSTNAME="beaglebone"
  BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS=y
  BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW=y

  # filesystem
  BR2_PACKAGE_AM33X_CM3=y
  BR2_TARGET_ROOTFS_EXT2=y
  BR2_TARGET_ROOTFS_EXT2_4=y

  # bootloader
  BR2_TARGET_BAREBOX=y
  BR2_TARGET_BAREBOX_BOARD_DEFCONFIG="am335x"
  BR2_TARGET_BAREBOX_IMAGE_FILE="images/barebox-am33xx-beaglebone.img"
  BR2_TARGET_BAREBOX_CUSTOM_ENV=y
  BR2_TARGET_BAREBOX_CUSTOM_ENV_PATH="board/beaglebone/barebox/barebox.env"
  BR2_TARGET_BAREBOX_AUX=y
  BR2_TARGET_BAREBOX_AUX_BOARD_DEFCONFIG="am335x_mlo"
  BR2_TARGET_BAREBOX_AUX_IMAGE_FILE="images/barebox-am33xx-beaglebone-mlo.img"

  # kernel
  BR2_LINUX_KERNEL=y
  BR2_LINUX_KERNEL_USE_DEFCONFIG=y
  BR2_LINUX_KERNEL_DEFCONFIG="omap2plus"
  BR2_LINUX_KERNEL_ZIMAGE=y

  # use the barebox built-in dtb
  # BR2_LINUX_KERNEL_DTS_SUPPORT is not set

Signed-off-by: Pieter Smith <pieter@boesman.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>