gnumake: Provide cc_rule and as_rule constructors

Use these to dynamically construct a rule for at32f4/fpec.o

Rules.mk

@@ -46,6 +46,18 @@ CFLAGS += $(CFLAGS-y) $(FLAGS) -include decls.h
 AFLAGS += $(AFLAGS-y) $(FLAGS) -D__ASSEMBLY__
 LDFLAGS += $(LDFLAGS-y) $(FLAGS) -Wl,--gc-sections
 
+define cc_rule
+$(1): $(SRCDIR)/$(2) $(SRCDIR)/Makefile
+	@echo CC $$@
+	$(CC) $$(CFLAGS) -c $$< -o $$@
+endef
+
+define as_rule
+$(1): $(SRCDIR)/$(2) $(SRCDIR)/Makefile
+	@echo AS $$@
+	$(CC) $$(AFLAGS) -c $$< -o $$@
+endef
+
 SRCDIR := $(shell $(PYTHON) $(ROOT)/scripts/srcdir.py $(CURDIR))
 include $(SRCDIR)/Makefile
 
@@ -93,12 +105,7 @@ endif
 	$(PYTHON) $(ROOT)/scripts/mk_update.py new $@ \
 	$< $(mcu)-$(FW_MAJOR).$(FW_MINOR)-$(bootloader)
 
-%.o: $(SRCDIR)/%.c $(SRCDIR)/Makefile
-	@echo CC $@
-	$(CC) $(CFLAGS) -c $< -o $@
-
-%.o: $(SRCDIR)/%.S $(SRCDIR)/Makefile
-	@echo AS $@
-	$(CC) $(AFLAGS) -c $< -o $@
+$(eval $(call cc_rule,%.o,%.c))
+$(eval $(call as_rule,%.o,%.S))
 
 -include $(DEPS)

src/mcu/at32f4/Makefile

@@ -2,5 +2,4 @@ OBJS += stm32.o
 OBJS += fpec.o
 OBJS += testmode.o
 
-fpec.c:
-	ln -sf ../stm32f1/$@ $@
+$(eval $(call cc_rule,fpec.o,../stm32f1/fpec.c))

src/mcu/at32f4/fpec.c

@@ -1 +0,0 @@
-../stm32f1/fpec.c