[build] Avoid unnecessary "rm" and "touch" in dependency generation

Speed up dependency generation by omitting the totally unnecessary
"rm" and "touch" commands.  This reduces the time taken to generate
dependencies by around 6%.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 
 	@$(ECHO) "  [DEPS] $(1)"
 	@$(MKDIR) -p $(dir $(2))
-	@$(RM) $(2)
-	@$(TOUCH) $(2)
 	@$(CPP) $(CFLAGS) $(CFLAGS_$(3)) $(CFLAGS_$(4)) -DOBJECT=$(4) \
 		-Wno-error -M $(1) -MG -MP | \
-		sed 's/\.o\s*:/_DEPS =/' >> $(2)
+		sed 's/\.o\s*:/_DEPS =/' > $(2)
 	@$(ECHO_E) '\n$$(BIN)/$(4).o :' \
 		 '$(1) $$(MAKEDEPS) $$(POST_O_DEPS) $$($(4)_DEPS)' \
 		 '\n\t$$(QM)$(ECHO) "  [BUILD] $$@"' \
 
 	@$(ECHO) "  [MEDIADEPS] $(1)"
 	@$(MKDIR) -p $(dir $(2))
-	@$(RM) $(2)
-	@$(TOUCH) $(2)
 	@$(ECHO_E) '$$(BIN)/%.$(1) : $$(BIN)/%.$(1).zbin' \
 		  '\n\t$$(QM)$(ECHO) "  [FINISH] $$@"' \
 		  '\n\t$$(Q)$$(CP) $$< $$@' \