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

[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>