dist/buildsystem_sanity_check: add check for CPU/CPU_MODEL migration

[cladmi]

Contribution description

Static tests to ensure.

• CPU Makefile.include/Makefile.features/Makefile.dep must not be
  included by the board directly anymore.

• CPU and CPU_MODEL variables must now be defined by
  boards/**/Makefile.features files instead of `**/Makefile.include.

• CPU is defined by board Makefile.features with a warning

This currently blacklist the slwstk6000b and cortexm.inc.mk that still define
'CPU_MODEL' in a file that is not a 'Makefile.features.
This allows getting this sanity check script in master

Depends on

• boards/slwstk6000b: move CPU definiton to Makefile.features boards/slwstk6000b: move CPU definiton to Makefile.features #12334
• boards: remove duplicate include 'RIOTCPU/cpu/Makefile.features' boards: remove duplicate include 'RIOTCPU/cpu/Makefile.features' #12335

This is currently only warning during build that CPU is not defined. I have commits in #11477 to enforce it, not sure if we want it already or not.

Testing procedure

The static tests do not fail, and no warning for any board regarding defining CPU.

When reverting both depending commits and removing the blacklist lines, the forbidden outputs are matched:

./dist/tools/buildsystem_sanity_check/check.sh
Invalid build system patterns found by ./dist/tools/buildsystem_sanity_check/check.sh:
Makefiles files from cpu must not be included by the board anymore
        boards/common/blxxxpill/Makefile.features:include $(RIOTCPU)/stm32f1/Makefile.features
        boards/p-l496g-cell02/Makefile.features:include $(RIOTCPU)/stm32l4/Makefile.features
        boards/samr34-xpro/Makefile.features:include $(RIOTCPU)/saml21/Makefile.features
        boards/slwstk6000b/Makefile.dep:include $(RIOTCPU)/efm32/Makefile.dep
        boards/slwstk6000b/Makefile.features:include $(RIOTCPU)/efm32/Makefile.features
        boards/stm32f723e-disco/Makefile.features:include $(RIOTCPU)/stm32f7/Makefile.features
CPU and CPU_MODEL definition must be done by board/BOARD/Makefile.features or board/common/**/Makefile.features
        boards/slwstk6000b/Makefile.include:export CPU = efm32
        boards/slwstk6000b/Makefile.include:export CPU_MODEL = $(MODULE_CPU)
        makefiles/arch/cortexm.inc.mk:export CPU_MODEL ?= $(CPU)

And the warning is triggered:

for board in $(make info-boards); do ESP32_SDK_DIR=/tmp ESP8266_SDK_DIR=/tmp ESP8266_NEWLIB_DIR=/tmp PORT=/dev/null BOARD=${board} make --n
o-print-directory -C examples/hello-world/ clean; done
/home/harter/work/git/RIOT/Makefile.features:18: CPU must be defined by board / board_common Makefile.features

Issues/PRs references

Depends on

• boards/slwstk6000b: move CPU definiton to Makefile.features boards/slwstk6000b: move CPU definiton to Makefile.features #12334
• boards: remove duplicate include 'RIOTCPU/cpu/Makefile.features' boards: remove duplicate include 'RIOTCPU/cpu/Makefile.features' #12335

Part of Tracking: move CPU/CPU_MODEL to Makefile.features #11477

[cladmi]

This one has overlap with #12338 as it fixes the need for a blacklist of cortexm.inc.mk. The one that goes second should remove the blacklist line.

[cladmi]

I rebased and adapted now that #12338 is merged.

I update the check as now CPU_MODEL is defined by cpu/CPU/Makefile.features for the cpu/lpc1768/Makefile.features.
It made sense since the beginning but was not required at first so was stricter.

The documentation and the test do not precise that CPU must be defined by the board and only the CPU_MODEL might be set by the cpu, but it would not make sense otherwise anyway. I could adapt if it is not clear enough.

[benpicco]

This adds a compile-time check to ensure all board Makefiles are following the standard.
Murdock just confirmed all boards pass that check.

This prevents new board additions from violating those standards.

[cladmi]

Thank you for the review.