[build] Ensure that isolinux.bin is modifiable

The -boot-info-table option to mkisofs will cause it to overwrite a
portion of the local copy of isolinux.bin.  Ensure that this file is
writable.

Originally-implemented-by: Nikolai Lifanov <lifanov@mail.lifanov.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/genfsimg

@@ -106,7 +106,7 @@ copy_syslinux_file() {
 	/usr/lib/ISOLINUX \
 	; do
 	if [ -e "${SRCDIR}/${FILENAME}" ] ; then
-	    cp "${SRCDIR}/${FILENAME}" "${DESTDIR}/"
+	    install -m 644 "${SRCDIR}/${FILENAME}" "${DESTDIR}/"
 	    return 0
 	fi
     done