cpu/cortexm: Add 'cortexm_fpu' as a DEFAULT_MODULE if possible

This adds cortexm_fpu to the DEFAULT_MODULE list when the feature
cortexm_fpu is provided by the architecture. It also moves the
dependency resolution of this module to the architecture-specific
Makefile.dep file.

cpu/cortexm_common/Makefile.dep

@@ -12,3 +12,9 @@ USEMODULE += newlib_nano
 
 # Export the peripheral drivers to be linked into the final binary:
 USEMODULE += periph
+
+# Use Hardware FPU by default if present
+FEATURES_OPTIONAL += cortexm_fpu
+ifneq (,$(filter cortexm_fpu,$(FEATURES_USED)))
+  DEFAULT_MODULE += cortexm_fpu
+endif

cpu/cortexm_common/Makefile.features

@@ -5,3 +5,8 @@ FEATURES_PROVIDED += periph_pm
 FEATURES_PROVIDED += cpp
 FEATURES_PROVIDED += cpu_check_address
 FEATURES_PROVIDED += ssp
+
+# cortex-m4f and cortex-m7 provide FPU support
+ifneq (,$(filter $(CPU_ARCH),cortex-m4f cortex-m7))
+  FEATURES_PROVIDED += cortexm_fpu
+endif

makefiles/arch/cortexm.inc.mk

@@ -60,24 +60,18 @@ endif # BUILD_IN_DOCKER
 CFLAGS += -DCPU_MODEL_$(call uppercase_and_underscore,$(CPU_MODEL))
 CFLAGS += -DCPU_ARCH_$(call uppercase_and_underscore,$(CPU_ARCH))
 
-# set the compiler specific CPU and FPU options
-ifneq (,$(filter $(CPU_ARCH),cortex-m4f cortex-m7))
-  ifneq (,$(filter cortexm_fpu,$(DISABLE_MODULE)))
-    CFLAGS_FPU ?= -mfloat-abi=soft
+# Add corresponding FPU CFLAGS
+# clang assumes there is an FPU, no CFLAGS necessary
+ifneq (llvm, $(TOOLCHAIN))
+  ifeq ($(CPU_ARCH),cortex-m7)
+    _CORTEX_HW_FPU_CFLAGS = -mfloat-abi=hard -mfpu=fpv5-sp-d16
   else
-    USEMODULE += cortexm_fpu
-    # clang assumes there is an FPU
-    ifneq (llvm,$(TOOLCHAIN))
-      ifeq ($(CPU_ARCH),cortex-m7)
-        CFLAGS_FPU ?= -mfloat-abi=hard -mfpu=fpv5-sp-d16
-      else
-        CFLAGS_FPU ?= -mfloat-abi=hard -mfpu=fpv4-sp-d16
-      endif
-    endif
+    _CORTEX_HW_FPU_CFLAGS = -mfloat-abi=hard -mfpu=fpv4-sp-d16
   endif
-else
-  CFLAGS_FPU ?= -mfloat-abi=soft
 endif
+# Add soft or hard FPU CFLAGS depending on the module
+# NOTE: This can be turned into normal conditional syntax once #9913 is fixed
+CFLAGS_FPU ?= $(if $(filter cortexm_fpu,$(USEMODULE)),$(_CORTEX_HW_FPU_CFLAGS),-mfloat-abi=soft)
 
 ifeq ($(CPU_ARCH),cortex-m4f)
   MCPU = cortex-m4