feat(nodediscover): set netboot=ipxe on discovered x86 nodes

A discovered x86 node without an x86 method got netboot=xnba, which
names the xNBA loader that xCAT retires. It now gets netboot=ipxe, the
method of the upstream iPXE loader that discovery itself uses. A node
that already has pxe, ipxe or xnba keeps it.

xCAT-server/lib/xcat/plugins/nodediscover.pm

@@ -147,7 +147,7 @@ sub _default_netboot {
     $currboot = '' unless defined $currboot;
 
     if ($arch =~ /x86/ and $currboot !~ /pxe/ and $currboot !~ /xnba/) {
-        return 'xnba';
+        return 'ipxe';
     } elsif ($arch =~ /ppc/ and $platform =~ /PowerNV/) {
         return 'petitboot';
     } elsif ($arch =~ /ppc/ and $currboot !~ /yaboot/) {