Actually locate UE install

inc/ueinstall.ps1

@@ -3,5 +3,27 @@ function Get-UE-Install {
         [string]$ueVersion
     )
 
-    return "TODO"
+    # UE4INSTALL env var should point at the root of the *specific version* of 
+    # UE4 you want to use. This is mainly for use in source builds, default is
+    # to build it from version number and root of all UE4 binary installs
+    $uinstall = $Env:UE4INSTALL
+
+    if (-not $uinstall) {
+        # UE4ROOT should be the parent folder of all UE versions
+        $uroot = $Env:UE4ROOT
+        if (-not $uroot) {
+            $uroot = "C:\Program Files\Epic Games"
+        } 
+
+        $uinstall = Join-Path $uroot "UE_$ueVersion"
+    }
+
+    # Test we can find RunUAT.bat
+    $batchfolder = Join-Path "$uinstall" "Engine\Build\BatchFiles"
+    $buildbat = Join-Path "$batchfolder" "RunUAT.bat"
+    if (-not (Test-Path $buildbat -PathType Leaf)) {
+        throw "RunUAT.bat missing at $buildbat : Not a valid UE install"
+    }
+
+    return $uinstall
 }