Overhaul scaninc to work recursively

This also fixes the bug where scaninc would ignore #include lines
in assembly files.

Makefile

@@ -168,7 +168,7 @@ $(ASM_BUILDDIR)/%.o: $(ASM_SUBDIR)/%.s $$(asm_dep)
 ifeq ($(NODEP),1)
 $(DATA_ASM_BUILDDIR)/%.o: data_dep :=
 else
-$(DATA_ASM_BUILDDIR)/%.o: data_dep = $(shell $(SCANINC) $(DATA_ASM_SUBDIR)/$*.s)
+$(DATA_ASM_BUILDDIR)/%.o: data_dep = $(shell $(SCANINC) -I include $(DATA_ASM_SUBDIR)/$*.s)
 endif
 
 $(DATA_ASM_BUILDDIR)/%.o: $(DATA_ASM_SUBDIR)/%.s $$(data_dep)