vexpress64: juno: add androidboot.hardware=juno

Linaro's Juno Android builds requires the androidboot.hardware parameter
be set to a know board name.

Non-Android kernels ignore this extra parameter because they don't
contain code to parse it.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>