Generate and use compiler dep rules

SDCC supports the `-MMD` option. Use it instead of having every file
depend on *all* the headers. Reduces incremental build time after
modifying headers as it will now only rebuild the actual dependents and
not the entire project.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/arch/8051/toolchain.mk

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-3.0-only
 
-CC = sdcc -mmcs51 --model-large --code-size $(CODE_SIZE) --xram-size $(SRAM_SIZE) --Werror
+CC = sdcc -mmcs51 -MMD --model-large --code-size $(CODE_SIZE) --xram-size $(SRAM_SIZE) --Werror
 
 AS = sdas8051
 ASFLAGS = -plosgff
@@ -48,3 +48,7 @@ $(ASM_OBJ): $(BUILD)/%.rel: src/%.asm
 $(C_OBJ): $(BUILD)/%.rel: src/%.c $(INCLUDE)
 	@mkdir -p $(@D)
 	$(CC) $(CFLAGS) -o $@ -c $<
+
+# Add dependency rules
+DEP = $(OBJ:%.rel=%.d)
+-include $(DEP)

src/arch/avr/toolchain.mk

@@ -1,7 +1,8 @@
 # SPDX-License-Identifier: GPL-3.0-only
 
-CC=avr-gcc -mmcu=$(EC_VARIANT)
-CFLAGS+=-Os -fstack-usage -Wall -Werror -Wl,--gc-sections -Wl,-u,vfprintf -lprintf_flt
+CC = avr-gcc -mmcu=$(EC_VARIANT)
+CFLAGS += -MMD -Os -fstack-usage -Wall -Werror \
+	-Wl,--gc-sections -Wl,-u,vfprintf -lprintf_flt
 
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105523
 ifneq ($(findstring 12.,$(shell avr-gcc --version 2>/dev/null)),)
@@ -35,3 +36,7 @@ $(BUILD)/ec.elf: $(OBJ)
 $(BUILD)/%.o: src/%.c $(INCLUDE)
 	@mkdir -p $(@D)
 	$(CC) $(CFLAGS) -o $@ -c $<
+
+# Add dependency rules
+DEP = $(OBJ:%.o=%.d)
+-include $(DEP)