TOP: Allow building platform out-of-tree

This patch extends our current build-system for building platform
sources which are not part of OpenSBI sources.

For example:

Let's say we have out-of-tree ABC platform sources. We place these
sources under <XYZ>/ABC directory along with its config.mk and
objects.mk.

To build out-of-tree ABC platform from OpenSBI directory:
$ make PLATFORM_DIR=<XYZ>/ABC
OR
$ make PLATFORM_DIR=<XYZ> PLATFORM=ABC

To build out-of-tree ABC platform from <XYZ>/ABC directory:
$ make PLATFORM_DIR=<XYZ>/ABC -C <path_to_opensbi>
OR
$ make PLATFORM_DIR=<XYZ> PLATFORM=ABC -C <path_to_opensbi>

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>