BaseTools: Update windows and linux run scripts file to use Python3

Modify windows script, PosixLike script, edksetup.sh, edksetup.bat to
use Python3 based on PYTHON3_ENABLE environment.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/BinWrappers/PosixLike/GenDepex

@@ -1,9 +1,9 @@
 #!/usr/bin/env bash
 #python `dirname $0`/RunToolFromSource.py `basename $0` $*
 
-# If a python2 command is available, use it in preference to python
-if command -v python2 >/dev/null 2>&1; then
-    python_exe=python2
+# If a ${PYTHON} command is available, use it in preference to python
+if command -v ${PYTHON} >/dev/null 2>&1; then
+    python_exe=${PYTHON}
 fi
 
 full_cmd=${BASH_SOURCE:-$0} # see http://mywiki.wooledge.org/BashFAQ/028 for a discussion of why $0 is not a good choice here