Add target use/bootloader/uboot. Das U-Boot -- the Universal Boot
Loader, used on single board PCs (also in qemu).

Use bootloader lists to provide better portability and to avoid
overcomplicating bootloader switch.

Added restriction for alterator-$$(BASE_BOOTLOADER)) only for
ISO images. Since the new uboot bootloader is not supported in
Alterator and is not used when building ISO images.