[build] Run parserom.pl only on potential driver files

PCI_ROM() and ISA_ROM() macros occur only within driver files.
Running parserom.pl on non-driver files is therefore redundant.

Skip running parserom.pl on any files outside a "drivers" directory.
This reduces the time taken to generate build rules and dependencies
after a "make veryclean" by around 12%.

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

src/Makefile.housekeeping

 		    '\n$(TGT)_OBJS += $$(BIN)/$(3).$(TGT)\n' ) ) \
 		 '\n$(BIN)/deps/$(1).d : $$($(3)_DEPS)\n' \
 		 '\nTAGS : $$($(3)_DEPS)\n' > $(BIN)/rules/$(1).r
-	@$(PERL) $(PARSEROM) $(1) >> $(BIN)/rules/$(1).r
+	@$(if $(findstring drivers/,$(1)),\
+	   $(PERL) $(PARSEROM) $(1) >> $(BIN)/rules/$(1).r)
 endef
 
 # Rule to generate the dependency list file