Dockerfile: add mkbootimg tool

mkbootimg tool is part of the Android project and it is
used to pack Android boot images such as boot image
and vendor_boot image.

Use the following command to run mkbootimg:
$ python3 -m mkbootimg

Add mkbootimg to the docker file

Signed-off-by: Safae Ouajih <souajih@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>