Initial version of the AArch64 Linux boot wrapper

This boot wrapper contains the code for initialising the ARMv8 software
model before the Linux kernel can run (see
Documentation/arm64/booting.txt in the kernel tree for the Linux booting
requirements).

Running "make" creates a "linux-system.axf" ELF file that can be loaded
by the software model.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>