Add mkdtboimg

Identical to the 'mkdtimg' written in C. The tool retains exactly the
same command line options but compacts the total code down to a single
compact python module.

'pymkdtimg' supports all commands supported by 'mkdtimg' except the
arguments cannot be device tree properties. That support depends on
'pylibfdt' being available.

The CL also adds test script that compares mkdtimg output with pymkdtimg
output using all supported commands.

TODO:
 - Add support for DT property arguments (depends on pylibfdt)
 - Rename tools to be called '[py]mkdtboimg', since they are actually
   creating the dtbo images
 - Test on a device

Test: cd utils/tests; ./mkdtboimg_testdata.sh

Change-Id: Id3919e634b08e2c642ca21769937e1277e3d2bc8
Signed-off-by: Sandeep Patil <sspatil@google.com>