defect 2948409: use subroutine getTftpDir to get the tftp dir

git-svn-id: https://svn.code.sf.net/p/xcat/code/xcat-core/trunk@5280 8638fb3e-16cb-4fca-ae20-7b5d299a9bcd

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

@@ -39,8 +39,7 @@ sub process_request {
       $sitetab->close;
    }
 
-   my $installdir = "/install";
-   my $tftpdir = "/tftpboot";
+   my $tftpdir = xCAT::Utils->getTftpDir();
    if (scalar(@{$request->{arg}}) != 1) {
       $callback->({error=>"Need to specifiy architecture (x86, x86_64 or ppc64)"},{errorcode=>[1]});
       return;