Makefile.inc: Add CARRIER_DIR to component discovery

The idea is to split the “mainboard” category into “variants” and
“carrierboards”, in the case when we use the COMe module together
with the Carrier Board instead of a single monolithic motherboard.
Previously, the “variants” category defined the type of motherboard,
which has a number of differences from the base one, for example, it
differed in the size or type of memory, and in the configuration of
the interfaces. Thus, there is no need to create a separate directory
in src/mainboard for a board that is similar in configuration to the
base board. But for a COMe module, “variants” contains different
variants of only this module, and the entire Carrier Board configuration
is allocated to a separate category - “carrierboards”, and each of the
variants can be used with one of the many boards in “carrierboards”.

For example, in the case of the Kontron mAL10 COMe module, variant
refers to the COMe-mAL10 or COMe-m4AL10 module type. They differ in the
type of memory (DDR3L or DDR4), and maybe they differ in some chips (see
more in https://www.kontron.com/products). However, all variants contain
the same type of processor/SoC.

The "carrierboards" directory can be able contain both the Kontron's
Evalution carrier boards (such as Eval Carrier2 T10 and COMe
Ref.Carrier-i T10  TNI) and third party vendor backplanes that are
compatible with the COMe modules from “variants”.

Thus, the src/mainboard/<module-name> directory contains the common
configuration code for all variants from src/mainboard/<module-name>/
variants, which can be supplemented/redefined with a configuration from
src/mainboard/<module-name>/carrierboard/<vendor-carrierboard-name>.

This architectural solution will be able to systematize and simplify
understanding of the code structure for COMe modules and will allow
vendors to add/maintain their code in a separate directory.

This work is also the first step towards to union of all carrierboards
into the global category in src/carrierboard on a par with all boards
from src/mainboard.

The patch takes this into account in the build system and adds
CARRIER_DIR component to use the “carrierboards” category, as it has
done for VARIANT_DIR.

TEST = Build ROM image for Kontron mAL10 COMe module together with T10
TNI carrier board (https://review.coreboot.org/c/coreboot/+/39133).

Change-Id: Ic6b2f8994b1293ae6f5bda8c9cc95128ba0abf7a
Signed-off-by: Maxim Polyakov <max.senia.poliak@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42609
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
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M Makefile.inc
1 file changed, 2 insertions(+), 1 deletion(-)