Add new wrapper emscons, which sets EMSCRIPTEN_TOOL_PATH in environment.

This way the Emscripten Tool no longer has to be copied to root build directory.

emscons

@@ -0,0 +1,20 @@
+#!/usr/bin/env python2
+
+'''
+Wrapping the scons invocation, EMSCRIPTEN_TOOL_PATH is set in the process
+environment, and can be used to locate the emscripten SCons Tool.
+Example:
+# Load emscripten Tool
+my_env = Environment(tools=['emscripten'], toolpath=[os.environ['EMSCRIPTEN_TOOL_PATH']])
+'''
+
+import os, subprocess, sys
+from tools import shared
+
+tool_path = os.path.join(shared.path_from_root('tools'), 'scons', 'site_scons', 'site_tools', 'emscripten')
+
+env = os.environ.copy()
+env[ 'EMSCRIPTEN_TOOL_PATH' ] = tool_path
+
+exit(subprocess.call(sys.argv[1:], env=env))
+