Allow bootstrap to be invoked via python2

This means that on most Linux distributions where python3 is the default, we
can invoke `python2 script/bootstrap.py` and have it all work

tools/atom_source_root.js

@@ -0,0 +1,3 @@
+var path = require('path')
+
+console.log(path.resolve(path.dirname(__dirname)))

tools/atom_source_root.py

@@ -1,9 +0,0 @@
-#!/usr/bin/env python
-
-import os
-
-"""Prints the absolute path of the root of atom-shell's source tree.
-"""
-
-
-print os.path.abspath(os.path.dirname(os.path.dirname(__file__)))

tools/get-endianness.js

@@ -0,0 +1 @@
+console.log(require('os').endianness() === 'BE' ? 'big' : 'little')

tools/linux/pkg-config-wrapper

@@ -31,6 +31,11 @@ then
   exit 1
 fi
 
+python2=$(which python2)
+if [ ! -x "$python2" ] ; then
+	python2=python
+fi 
+
 rewrite=`dirname $0`/rewrite_dirs.py
 package=${!#}
 
@@ -46,4 +51,4 @@ set -e
 # pkg-config's |prefix| variable.
 prefix=`PKG_CONFIG_LIBDIR=$libdir pkg-config --variable=prefix "$package" | sed -e 's|/usr$||'`
 result=`PKG_CONFIG_LIBDIR=$libdir pkg-config "$@"`
-echo "$result"| $rewrite --sysroot "$root" --strip-prefix "$prefix"
+echo "$result"| $python2 $rewrite --sysroot "$root" --strip-prefix "$prefix"