siemens/mc_apl1: Provide baseboard and variant concepts

Siemens will provide further boards based on Apollo Lake. These differ
only slightly. To avoid copying the complete directory of the reference
board we simply create variants that only contain the differences, like
google/reef does.

To further the ability of multiple variant boards to share code provide
a place to land the split-up changes. This patch provides the tooling
by using a new Kconfig value, VARIANT_DIR, as well as the Make plumbing.
The directory layout with a single variant mc_apl1 (which is also the
baseboard) looks like this:

variants/baseboard - code
variants/baseboard/include/baseboard - headers
variants/mc_apl1 - code
variants/mc_apl1/include/variant - headers

New boards would then be added under their board name within the
'variants' directory.

No split has been done with providing different logic yet. This is
purely an organizational change.

Change-Id: Ia3c1f45daee3b9690a448b82edbeec552ee05973
Signed-off-by: Mario Scheithauer <mario.scheithauer@siemens.com>
Reviewed-on: https://review.coreboot.org/25785
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

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