BaseTools: Add YAML files with path env and tool extdeps

https://bugzilla.tianocore.org/show_bug.cgi?id=2315

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Bin/iasl_ext_dep.yaml

@@ -0,0 +1,21 @@
+## @file
+# Download iasl executable tool from a nuget.org package
+# - package contains different binaries based on host
+# Add the folder with the tool to the path
+#
+# This is only downloaded for scope cibuild thus
+# should have no impact on the asl compiler used by any
+# given platform to build.
+#
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: BSD-2-Clause-Patent
+##
+{
+  "id": "iasl-ci-1",
+  "scope": "cibuild",
+  "type": "nuget",
+  "name": "iasl",
+  "source": "https://api.nuget.org/v3/index.json",
+  "version": "20190215.0.0",
+  "flags": ["set_path", "host_specific"]
+}