[build] Search for ldlinux.c32 separately from isolinux.bin

Some distributions (observed with Ubuntu 15.04) place ldlinux.c32 in a
separate directory from isolinux.bin.  Search for these files
separately, and allow an alternative location of ldlinux.c32 to be
provided via LDLINUX_C32=... on the make command line.

Reported-by: Adrian Koshka <adriankoshcha@teknik.io>
Tested-by: Adrian Koshka <adriankoshcha@teknik.io>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/geniso

@@ -123,8 +123,7 @@ case "${LEGACY}" in
 		cp ${ISOLINUX_BIN} ${dir}
 
 		# syslinux 6.x needs a file called ldlinux.c32
-		LDLINUX_C32=$(dirname ${ISOLINUX_BIN})/ldlinux.c32
-		if [ -s ${LDLINUX_C32} ]; then
+		if [ -n "${LDLINUX_C32}" -a -s "${LDLINUX_C32}" ]; then
 			cp ${LDLINUX_C32} ${dir}
 		fi