Add OpenBSD 5.9

Drops 5.7 since it's not in the path anymore

src/openbsd.ipxe

@@ -5,13 +5,13 @@
 
 :openbsd_menu
 menu OpenBSD
+item 5.9 OpenBSD 5.9
 item 5.8 OpenBSD 5.8
-item 5.7 OpenBSD 5.7
 item snapshots OpenBSD 5.9 Latest Snapshot
 choose ver || goto openbsd_exit
 
+iseq ${ver} 5.9 && set image_ver 59 ||
 iseq ${ver} 5.8 && set image_ver 58 ||
-iseq ${ver} 5.7 && set image_ver 57 ||
 iseq ${ver} snapshots && set image_ver 59 ||
 
 iseq ${arch} x86_64 && goto openbsd_x64 ||