Makefile.dep: process cpu dependencies and fix stm32_common

[cladmi]

Contribution description

This adds processing cpu dependencies and fixes for periph_flashpage with stm32_common.
The Makefile.dep include from periph/Makefile.dep has also been removed.

I can split them in different PRs if wanted.

I added a test commit that should be removed to show the problem.

Testing procedure

Compiling the given test is fixed by the PR, it can be tested with the first commit alone:

make -C tests/periph_flash_test_stm32_common/ BOARD=iotlab-m3  RIOT_CI_BUILD=1

make: Entering directory '/home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common'
Building application "tests_periph_flash_test_stm32_common" for "iotlab-m3" with MCU "stm32f1".

/usr/lib/gcc/arm-none-eabi/8.2.0/../../../../arm-none-eabi/bin/ld: /home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common/bin/iotlab-m3/stm32_common_periph.a(flashpage.o): in function `_unlock_flash':
/home/cladmi/git/work/RIOT/cpu/stm32_common/periph/flashpage.c:58: undefined reference to `_unlock'
/usr/lib/gcc/arm-none-eabi/8.2.0/../../../../arm-none-eabi/bin/ld: /home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common/bin/iotlab-m3/stm32_common_periph.a(flashpage.o): in function `flashpage_write_raw':
/home/cladmi/git/work/RIOT/cpu/stm32_common/periph/flashpage.c:202: undefined reference to `_lock'
/usr/lib/gcc/arm-none-eabi/8.2.0/../../../../arm-none-eabi/bin/ld: /home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common/bin/iotlab-m3/stm32_common_periph.a(flashpage.o): in function `_unlock_flash':
/home/cladmi/git/work/RIOT/cpu/stm32_common/periph/flashpage.c:58: undefined reference to `_unlock'
/usr/lib/gcc/arm-none-eabi/8.2.0/../../../../arm-none-eabi/bin/ld: /home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common/bin/iotlab-m3/stm32_common_periph.a(flashpage.o): in function `_erase_page':
/home/cladmi/git/work/RIOT/cpu/stm32_common/periph/flashpage.c:142: undefined reference to `_lock'
collect2: error: ld returned 1 exit status
make: *** [/home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common/../../Makefile.include:431: /home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common/bin/iotlab-m3/tests_periph_flash_test_stm32_common.elf] Error 1
make: Leaving directory '/home/cladmi/git/work/RIOT/tests/periph_flash_test_stm32_common'

Also, the stm32_common/periph/i2c_X handling is still working:

This works both in master and with this PR

make -C tests/periph_i2c/ BOARD=iotlab-m3
make -C tests/periph_i2c/ BOARD=nucleo-f070rb

ar t tests/periph_i2c/bin/nucleo-f070rb/stm32_common_periph.a  | grep 'i2c_..o'
i2c_1.o
ar t tests/periph_i2c/bin/iotlab-m3/stm32_common_periph.a  | grep 'i2c_2.o'
i2c_2.o

Issues/PRs references

Found in #9342 main PR
Part of the main issue "dependencies declared in cpu/board Makefile.include" #8713

[cladmi]

The approach cannot work because in

RIOT/makefiles/info-global.inc.mk

Line 27 in 8d1d509

include $(RIOTBASE)/Makefile.dep

the CPU is not known, so it should be done by the board Makefile.dep with hardcoded cpu.

I will do another PR in this direction.

[cladmi]

Hardwriting this would mean re-adding again some hardwriten duplication of the CPU variable, I will try something addressing it.