If CONFIG_RELOCATABLE is enabled, call relocate_kernel.

This function will return the entry point of the relocated kernel if
copy/relocate is sucessful or the original entry point if not. The stack
pointer must then be pointed into the new image.
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: kernel-hardening@lists.openwall.com
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12984/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>