[console] Ignore unexpected keysyms when generating keyboard maps

I am unable to find any definitive documentation on how Linux keyboard
symbols work.  In the absence of any documentation, I'm going to
assume that unexpected keysyms are harmless and should be ignored.

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

src/util/genkeymap.pl

   return unless $keysym;
 
   # Sanity check
-  die "Unexpected keysym ".sprintf ( "0x%04x\n", $keysym )."\n"
-      if $keysym & 0xf000;
+  if ( $keysym & 0xf000 ) {
+    warn "Unexpected keysym ".sprintf ( "0x%04x", $keysym )."\n";
+    return;
+  }
 
   # Extract type and value
   my $type = ( $keysym >> 8 );