[build] Avoid invoking genkeymap.py via Perl

The build process currently invokes the Python genkeymap.py script via
the Perl executable.  Strangely, this appears to work.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

@@ -24,6 +24,7 @@ CP		:= cp
 ECHO		:= echo
 PRINTF		:= printf
 PERL		:= perl
+PYTHON		:= python
 TRUE		:= true
 CC		:= $(CROSS_COMPILE)gcc
 CPP		:= $(CC) -E