Custom.master definition in ONET.XML file

<ListTemplates>

<ListTemplate Name="mplib" DisplayName="$Resources:MasterPageGallery;" Description="$Resources:global_onet_mplib_desc;" SetupPath="global\lists\mplib" Type="116" BaseType="1" Path="GLOBAL" Hidden="TRUE" HiddenList="TRUE" NoCrawl="TRUE" Unique="TRUE" Catalog="TRUE" OnQuickLaunch="FALSE" SecurityBits="11" AllowDeletion="FALSE" AllowEveryoneViewItems="TRUE" Image="/_layouts/images/itdl.gif" AlwaysIncludeContent="TRUE" DocumentTemplate="100" />

</ListTemplates>

<Configuration ID="1" Name="Blank" MasterUrl="_catalogs/masterpage/custom.master">

     <Lists />

     <Modules>

       <Module Name="DefaultBlank" />

<Module Name="CustomMasterPage" />

     </Modules>

     <SiteFeatures>

       <!-- BasicWebParts Feature -->

       <Feature ID="00BFEA71-1C5E-4A24-B310-BA51C3EB7A57" />

     </SiteFeatures>

     <WebFeatures>

       <!-- TeamCollab Feature -->

       <Feature ID="00BFEA71-4EA5-48D4-A4AD-7EA5C011ABE5" />

     </WebFeatures>

   </Configuration>

   </Module>

     <Module Name="CustomMasterPage" List="116" Url="_catalogs/masterpage" RootWebOnly="FALSE">

     <File Url="custom.master" Type="GhostableInLibrary" IgnoreIfAlreadyExists="TRUE" />

   </Module>