Boot: Add Python2.7 support for post build steps

These minor changes make Python 3 and Python 2.7 verions supported by
the post build scripts, even integrating them as modules.

Change-Id: I0d7a6d95764db082bbec02b0af01da4cf03fd98d
Signed-off-by: Gabor Kertesz <gabor.kertesz@arm.com>
diff --git a/bl2/ext/mcuboot/scripts/imgtool.py b/bl2/ext/mcuboot/scripts/imgtool.py
index 4b62d3b..80009c8 100644
--- a/bl2/ext/mcuboot/scripts/imgtool.py
+++ b/bl2/ext/mcuboot/scripts/imgtool.py
@@ -15,12 +15,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import print_function
 import os
 import re
 import argparse
-from imgtool import keys
-from imgtool import image
-from imgtool import version
+from imgtool_lib import keys
+from imgtool_lib import image
+from imgtool_lib import version
 import sys
 
 def find_load_address(args):