arm: vexpress64: juno: Enable distro_bootcmd functionality

The ARM Juno boards can be used as somewhat decent machines to run
off-the-shelf distributions, with USB, SATA, GBit Ethernet and 8GB of
DRAM.

With stable DTs in the board's NOR flash this would work really nicely,
however the default boot command is to fetch a kernel and an initrd from
the on-board NOR flash, which sounds somewhat embedded.

Include the config_distro_bootcmd.h header and define the available
devices (starting with USB, to catch USB installer sticks) to make
distributions and UEFI work out of the box.
The NOR flash kernel functionality is still preserved as the last
resort, should all other methods fail.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>