main/avbtool: new aport

This tool is used to manipulate Android Verified Boot 2.0 structures. It can be used in new Android devices (>= Android 8, which require AVB) to create vbmeta.img file with AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED flag, to be flashed to vbmeta partition to disable verified boot checks.

There is no C binary program to work with avb meta; there is only one upstream python script, which ony works with python2. Luckily 2to3 tool + some manual editing can help to make it compatible with python3.