[build] Update build system for Syslinux 6.x

Syslinux 6.x places its files into a bios subdirectory, and requires
that a ldlinux.c32 module be included within the ISO image.  Add the
relevant search paths for isolinux.bin, and include the file
ldlinux.c32 within the ISO image if it exists.

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

src/arch/i386/Makefile

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

src/util/geniso

 dir=`mktemp -d bin/iso.dir.XXXXXX`
 cfg=$dir/isolinux.cfg
 cp -p $isolinux_bin $dir
+
+# syslinux 6.x needs a file called ldlinux.c32
+ldlinux_c32=$(dirname ${isolinux_bin})/ldlinux.c32
+if [ -s ${ldlinux_c32} ]
+then
+	cp -p ${ldlinux_c32} ${dir}
+fi
+
 cat > $cfg <<EOF
 # These default options can be changed in the geniso script
 SAY iPXE ISO boot image