mtk-bootloader-tool.py: Tool to interact with bootloader/download agent.

Currently, can read ROM contents on MT6580 based devices (and has values
hardcoded for it and particular download agent version). This tool is
inspired by earlier project https://github.com/chris1uk/mtk-linux
(and uses command names from it), but rewritten from scratch with the
hope to make it cleaner and more general.