[pxe] Fill in UNDIROMID pointer in !PXE structure

IBM's iSCSI Firmware Initiator checks the UNDIROMID pointer in the
!PXE structure that gets created by the UNDI loader.  We didn't
previously fill this value in.

src/arch/i386/scripts/i386.lds

@@ -30,6 +30,16 @@ SECTIONS {
      *
      */
 
+    /*
+     * Weak symbols that need zero values if not otherwise defined
+     */
+
+    . = 0;
+    .weak : AT ( 0 ) {
+	*(.weak)
+    }
+    _assert = ASSERT ( ( . == 0 ), ".weak is non-zero length" );
+
     /*
      * The prefix
      */