armv8r64-extras/zephyr: Make number of CPUs configurable

This patch makes the number of CPUs used by Zephyr applications
configurable using the variable ZEPHYR_NUM_CPUS.

This affects the CONFIG_MP_NUM_CPUS Zephyr config flag, as well as the
cluster0.NUM_CORES parameter of the FVP.

For the fvp-baser-aemv8r64, the Zephyr repository contains separate SMP
and non-SMP defconfig files. Move the Zephyr machine configuration from
the machine conf file to the bbappend so this can be modified on a
per-app basis. Dynamically set ZEPHYR_BOARD based on ZEPHYR_NUM_CPUS.

Issue-Id: SCM-4697
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Icf82cca4fbe4c8b082479b1c6ab6a54f44507f44