cpu/cortexm_common: enable custom newlib syscalls w/ newlib_syscalls_X

Makefile.dep

@@ -270,6 +270,11 @@ ifneq (,$(filter gnrc_pktdump,$(USEMODULE)))
 endif
 
 ifneq (,$(filter newlib,$(USEMODULE)))
+  # allow custom newlib syscalls implementations by adding
+  # newlib_syscalls_XXX to USEMODULE
+  ifeq (,$(filter newlib_syscalls_%,$(USEMODULE)))
+    USEMODULE += newlib_syscalls_default
+  endif
   USEMODULE += uart_stdio
 endif
 

sys/Makefile.include

@@ -61,7 +61,7 @@ ifneq (,$(filter log_%,$(USEMODULE)))
     include $(RIOTBASE)/sys/log/Makefile.include
 endif
 
-ifneq (,$(filter newlib,$(USEMODULE)))
+ifneq (,$(filter newlib_syscalls_default,$(USEMODULE)))
     include $(RIOTBASE)/sys/newlib/Makefile.include
 endif
 

sys/newlib/Makefile

+MODULE = newlib_syscalls_default
+
 include $(RIOTBASE)/Makefile.base

sys/newlib/Makefile.include

-UNDEF := $(BINDIR)newlib/syscalls.o $(UNDEF)
+UNDEF := $(BINDIR)newlib_syscalls_default/syscalls.o $(UNDEF)
 
 # Search for Newlib include directories