vexpress64: Clean up BASE_FVP boot configuration

Move env var address values to #defines so they can be reused elsewhere.

Rename env var names to those recommended in the README and modify
addresses to allow more space for the kernel.

Fix issue where fdt is called with invalid arguments when booting
without a ramdisk.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>