[build] Avoid hard-coding the path to perl

[build] Avoid hard-coding the path to perl

The path "/usr/bin/perl" has been hard-coded since Etherboot 5.1, for
no discernible reason.  Use just "perl" instead to fix the
inconsistency and allow building on systems with Perl installed
outside of /usr/bin.

This commit also includes a later fix that removes a dependency on
"perl" which broke builds from fully clean trees.

Reported-by: Gabor Z. Papp <gzp@papp.hu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>