[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.

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

src/Makefile

 CP		:= cp
 ECHO		:= echo
 PRINTF		:= printf
-PERL		:= /usr/bin/perl
+PERL		:= perl
 CC		:= $(CROSS_COMPILE)gcc
 CPP		:= $(CC) -E
 AS		:= $(CROSS_COMPILE)as