$scope.currSel = null;
$scope.metadata = [
    { proc: "Proc-1", board: "Alloc" },
    { proc: "Proc-1", board: "Manager" },
    { proc: "Proc-1", board: "Ops" },
    { proc: "Proc-2", board: "Alloc" },
    { proc: "Proc-2", board: "Manager" },
    { proc: "Proc-3", board: "Alloc" },
    { proc: "Proc-3", board: "Manager" }
];

<select ng-model="currSel"
        ng-options="item.board group by item.proc
                        for item in metadata">
</select>

for ( var i in $scope.metadata ) {
    var tmp = $scope.metadata[i];
    tmp.displayAs = [ tmp.proc, tmp.board ].join("/");
}

<select ng-model="currSel"
        ng-options="item.displayAs
                    group by item.proc for item in metadata">
</select>