[build] Add FreeBSD location for isolinux

Reported-by: Jedrzej Kalinowski <kalinoj1@iem.pw.edu.pl>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/Makefile

 ISOLINUX_BIN_LIST	:= \
 	$(ISOLINUX_BIN) \
 	/usr/lib/syslinux/isolinux.bin \
-	/usr/share/syslinux/isolinux.bin
+	/usr/share/syslinux/isolinux.bin \
+	/usr/local/share/syslinux/isolinux.bin
 ISOLINUX_BIN	= $(firstword $(wildcard $(ISOLINUX_BIN_LIST)))
 
 # i386-specific directories containing source files