vexpress64: juno: add default NOR flash boot

This modifies the vexpress64 Juno configuration so that
it will by default load and boot a kernel and a device tree
from the images stored in the NOR flash. When we are
at it, also define the proper command line for the Juno and
indicate that the USB stick (/dev/sda1) is the default
root file system.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>