cpu/stm32_common: remove inadapted periph_flash_common

The `periph_flash_common` feature was only defined here to trigger
inclusion of a source file with common functions.
It even only defines private symbols `_lock` and `_unlock` so no reason
to expose it to the build system.
And in practice, all stm cpus providing `periph_flashpage` or
`periph_eeprom` were required to provide `periph_flash_common` to allow
including it.

The previous implementation was only parsing in the modules were in
`FEATURES_REQUIRED` wich did not take cases of `FEATURES_OPTIONAL` into
account.
And also, in the same time, as the dependencies was declared in
`Makefile.include` it was processed before `Makefile.dep` so never handled
cases where a module could depend on `periph_flashpage` or
`periph_eeprom` feature.

It is replaced by selecting the common source file when module using it
are included.

The now useless feature `periph_flash_common` is removed from
`FEATURES_PROVIDED`.

cpu/stm32_common/Makefile.include

@@ -8,12 +8,6 @@ USEMODULE += periph_common
 # include stm32 common functions and stm32 common periph drivers
 USEMODULE += stm32_common stm32_common_periph
 
-# flashpage and eeprom periph implementations share flash lock/unlock functions
-# in periph_flash_common
-ifneq (,$(filter periph_flashpage periph_eeprom,$(FEATURES_REQUIRED)))
-  FEATURES_REQUIRED += periph_flash_common
-endif
-
 # For stm32 cpu's we use the stm32_common.ld linker script
 export LINKFLAGS += -L$(RIOTCPU)/stm32_common/ldscripts
 LINKER_SCRIPT ?= stm32_common.ld

cpu/stm32_common/periph/Makefile

@@ -9,4 +9,10 @@ ifneq (,$(filter periph_i2c,$(USEMODULE)))
   endif
 endif
 
+# flashpage and eeprom periph implementations share flash lock/unlock functions
+# defined in flash_common.c
+ifneq (,$(filter periph_flashpage periph_eeprom,$(USEMODULE)))
+  SRC += flash_common.c
+endif
+
 include $(RIOTMAKE)/periph.mk

cpu/stm32f0/Makefile.features

 ifeq (,$(filter nucleo-f031k6,$(BOARD)))
-  FEATURES_PROVIDED += periph_flash_common
   FEATURES_PROVIDED += periph_flashpage
   FEATURES_PROVIDED += periph_flashpage_raw
 endif

cpu/stm32f1/Makefile.features

-FEATURES_PROVIDED += periph_flash_common
 FEATURES_PROVIDED += periph_flashpage
 FEATURES_PROVIDED += periph_flashpage_raw
 

cpu/stm32l0/Makefile.features

 FEATURES_PROVIDED += periph_eeprom
-FEATURES_PROVIDED += periph_flash_common
 FEATURES_PROVIDED += periph_flashpage
 FEATURES_PROVIDED += periph_flashpage_raw
 FEATURES_PROVIDED += periph_hwrng

cpu/stm32l1/Makefile.features

 FEATURES_PROVIDED += periph_eeprom
-FEATURES_PROVIDED += periph_flash_common
 FEATURES_PROVIDED += periph_flashpage
 FEATURES_PROVIDED += periph_flashpage_raw
 

cpu/stm32l4/Makefile.features

-FEATURES_PROVIDED += periph_flash_common
 FEATURES_PROVIDED += periph_flashpage
 FEATURES_PROVIDED += periph_flashpage_raw
 FEATURES_PROVIDED += periph_hwrng