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/board/system76/common/scratch/scratch.mk

@@ -15,7 +15,7 @@ SCRATCH_CFLAGS=$(CFLAGS)
 
 # Include scratch source
 SCRATCH_DIR=$(SYSTEM76_COMMON_DIR)/scratch
-SCRATCH_INCLUDE+=$(wildcard $(SCRATCH_DIR)/include/scratch/*.h) $(SCRATCH_DIR)/scratch.mk
+SCRATCH_INCLUDE += $(SCRATCH_DIR)/scratch.mk
 SCRATCH_CFLAGS+=-I$(SCRATCH_DIR)/include -D__SCRATCH__
 SCRATCH_SRC += $(foreach src, $(scratch-y), $(SCRATCH_DIR)/$(src))
 
@@ -24,6 +24,7 @@ SCRATCH_OBJ=$(sort $(patsubst src/%.c,$(SCRATCH_BUILD)/%.rel,$(SCRATCH_SRC)))
 SCRATCH_CC=\
 	sdcc \
 	-mmcs51 \
+	-MMD \
 	--model-small \
 	--code-loc $(SCRATCH_OFFSET) \
 	--code-size $(SCRATCH_SIZE) \