[build] Fix broken build caused by implied dependency upon "perl"

Commit ea12dc0 ("[build] Avoid hard-coding the path to perl")
introduced a build failure for fully clean trees (e.g. after running
"make veryclean"), since the dependency upon $(PARSEROM) now includes
a dependency upon "perl" (which doesn't exist) rather than upon
"/usr/bin/perl" (which does exist).

There should of course be no dependency upon the perl binary at all;
the dependency should be upon "./util/parserom.pl" alone.

Fix by removing the $(PERL) from the definition of Perl-based utility
paths, and adding $(PERL) at the point of usage.

Reported-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

@@ -486,7 +486,7 @@ define src_template
 		 '\n$(2) : $$($(4)_DEPS)\n' \
 		 '\nTAGS : $$($(4)_DEPS)\n' \
 		>> $(2)
-	@$(PARSEROM) $(1) >> $(2)
+	@$(PERL) $(PARSEROM) $(1) >> $(2)
 
 endef
 
@@ -701,7 +701,7 @@ $(BIN)/%.tmp : $(BLIB) $(MAKEDEPS) $(LDSCRIPT)
 	$(QM)$(ECHO) "  [LD] $@"
 	$(Q)$(LD) $(LDFLAGS) -T $(LDSCRIPT) $(TGT_LD_FLAGS) $(BLIB) -o $@ \
 		--defsym _build_id=`$(BUILD_ID_CMD)` -Map $(BIN)/$*.tmp.map
-	$(Q)$(OBJDUMP) -ht $@ | $(SORTOBJDUMP) >> $(BIN)/$*.tmp.map
+	$(Q)$(OBJDUMP) -ht $@ | $(PERL) $(SORTOBJDUMP) >> $(BIN)/$*.tmp.map
 
 # Keep intermediate object file (useful for debugging)
 .PRECIOUS : $(BIN)/%.tmp
@@ -758,7 +758,7 @@ $(BIN)/%.licence : $(BIN)/%.tmp
 		echo "files are missing a licence declaration:" ;\
 		echo $(call unlicensed_deps_list,$<);\
 		exit 1,\
-		$(LICENCE) $(call licence_list,$<))
+		$(PERL) $(LICENCE) $(call licence_list,$<))
 
 # Extract compression information from intermediate object file
 #
@@ -986,7 +986,7 @@ $(SYMTAB) : $(BLIB)
 CLEANUP	+= $(BIN)/symtab
 
 symcheck : $(SYMTAB)
-	$(SYMCHECK) $<
+	$(PERL) $(SYMCHECK) $<
 
 endif # defined(BIN)