rework Makefile

One issue is that dependencies are not properly generated, this will be
implemented later.

Makefile

@@ -1,44 +1,59 @@
 SHELL = /bin/sh
-.SUFFIXES:
+.SUFFIXES: # Disable implicit rules.
 
 CC        = avr-gcc
-STRIP  = avr-strip
+CFLAGS   := $(CFLAGS) -mmcu=avr128da28 -Os -std=gnu99 -Wall -Wextra -Wpedantic -Wno-pointer-arith
+CPPFLAGS := $(CPPFLAGS) -Iinclude -DNDEBUG
 
-CPPFLAGS = -DNDEBUG
-CFLAGS   = -Os
+ASFLAGS  := $(ASFLAGS) -mmcu=avr128da28 -Os
+LDFLAGS  := $(LDFLAGS) -nostdlib -flto
+LDLIBS   := $(LDLIBS)  -lgcc
 
-CPPFLAGS += -Iinclude
-CFLAGS   += -mmcu=avr128da28 -std=gnu99 -MMD -MP
-LDFLAGS  += -mmcu=avr128da28 -Wall -Wextra -Wpedantic -Wno-pointer-arith
-LDFLAGS  += -nostdlib -nostartfiles -flto
+# Find source files
+SRC := $(shell find src/ -name '*.[cS]' -print)
+DIR := $($(sort $(foreach f,$(SRC),$(dir $f))):%=obj/%) bin/
 
-SRC := $(wildcard src/*.c src/*/*.c src/*/*/*.c src/*/*/*/*.c src/*/*/*/*/*.c src/*/*/*/*/*/*.c src/*/*/*/*/*/*/*.c src/*/*/*/*/*/*/*/*.c)
-SRC += $(wildcard src/*.S src/*/*.S src/*/*/*.S src/*/*/*/*.S src/*/*/*/*/*.S src/*/*/*/*/*/*.S src/*/*/*/*/*/*/*.S src/*/*/*/*/*/*/*/*.S)
-OBJ := $(SRC:%=obj/%.o)
+# Configure common functions for logging
+msg-as    = $(info	[AS]	$(1))
+msg-cc    = $(info	[CC]	$(1))
+msg-clean = $(info	[CLEAN]	$(1))
+msg-ld    = $(info	[LD]	$(1))
+msg-tar   = $(info	[TAR]	$(1))
+msg-xxd   = $(info	[XXD]	$(1))
 
+# Set Q to @ to silence commands being printed, if --no-silent has not been set
+ifeq (0, $(words $(findstring --no-silent,$(MAKEFLAGS))))
+Q=@
+endif
 
-.PHONY: all clean install-strip info
-all: info bin/a.hex
-clean:; @-$(RM) -rv bin/ obj/
-info:
-	$(info CC:       $(CC))
-	$(info CFLAGS:   $(CFLAGS))
-	$(info CPPFLAGS: $(CPPFLAGS))
-	$(info LDFLAGS:  $(LDFLAGS))
-	$(info LDLIBS:   $(LDLIBS))
+.PHONY: $(DIR) all clean
+all: bin/a.out bin/a.hex
+clean:; $(Q)-$(RM) -rv bin/ obj/
 
-bin/a.out: $(OBJ)
-	$(info [LD]	$(CC)	$@)
-	@mkdir -p $(@D)
-	@$(CC) -o $@ $^ $(LDFLAGS) $(LDLIBS)
+# Links object files to a binary
+bin/a.out: $(SRC:%=obj/%.o)
+	$(Q)$(call msg-ld,$@)
+	$(Q)$(CC) $(LDFLAGS) $(LDLIBS) -o $@ $^
 
-bin/a.hex: bin/a.out
-	$(info [STRIP]	$(STRIP)	$@)
-	@$(STRIP) -o $@ $<
+# Links object files to a stripped binary
+bin/a.hex: $(SRC:%=obj/%.o)
+	$(Q)$(call msg-ld,$@)
+	$(Q)$(CC) -s $(LDFLAGS) $(LDLIBS) -o $@ $^
 
-obj/%.o: %
-	$(info [CC]	$(CC)	$@)
-	@mkdir -p $(@D)
-	@$(CC) -c $(CPPFLAGS) $(CFLAGS) -o $@ $<
+# Assembles preprocessed assembly into object files
+obj/%.S.d: %.S;  $Q$(CC) -MM $(ASFLAGS) $(CPPFLAGS) -MF $@ $<
+obj/%.S.o: %.S
+	$(Q)$(call msg-as,$@)
+	$(Q)$(CC) -c $(ASFLAGS) $(CPPFLAGS) -o $@ $<
 
--include $(OBJ:%.o=%.d)
+# Compiles C sources into object files
+obj/%.c.d: %.c;  $Q$(CC) -MM $(CFLAGS) $(CPPFLAGS) -MF $@ $<
+obj/%.c.o: %.c %.c.d
+	$(Q)mkdir $(@D)
+	$(Q)$(call msg-cc,$@)
+	$(Q)$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
+
+# Include dependencies, ignoring any errors that may occur when doing so.
+ifeq (0, $(words $(findstring $(MAKECMDGOALS), clean)))
+# -include $(SRC:%=obj/%.d)
+endif