[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

@@ -31,12 +31,12 @@ RANLIB		:= $(CROSS_COMPILE)ranlib
 OBJCOPY		:= $(CROSS_COMPILE)objcopy
 NM		:= $(CROSS_COMPILE)nm
 OBJDUMP		:= $(CROSS_COMPILE)objdump
-PARSEROM	:= $(PERL) ./util/parserom.pl
-FIXROM		:= $(PERL) ./util/fixrom.pl
-SYMCHECK	:= $(PERL) ./util/symcheck.pl
-SORTOBJDUMP	:= $(PERL) ./util/sortobjdump.pl
-PADIMG		:= $(PERL) ./util/padimg.pl
-LICENCE		:= $(PERL) ./util/licence.pl
+PARSEROM	:= ./util/parserom.pl
+FIXROM		:= ./util/fixrom.pl
+SYMCHECK	:= ./util/symcheck.pl
+SORTOBJDUMP	:= ./util/sortobjdump.pl
+PADIMG		:= ./util/padimg.pl
+LICENCE		:= ./util/licence.pl
 NRV2B		:= ./util/nrv2b
 ZBIN		:= ./util/zbin
 ELF2EFI32	:= ./util/elf2efi32