[Do Not Merge] Shell improvement, serial device abstraction, log message ...

[msplr]

This PR contains changes I wanted to do a while ago and finally got around to start working on:

• Improved shell with character echo, command arguments and better way to add new commands
• SerialDevice abstraction for reading/writing from/to UART, memory buffer. (similar to a FILE handle)
• prettier message logging over debug UART (wip)
• improve concurrent UART access from several threads. (tbd)
• general code cleanup

I'm placing it here to start the discussion. Feel free to review and comment. Please do not merge (yet).

[bitmorse]

Reviewed, all good, but yet to be tested. Ready to get rid of cli_printf. Would also like to use cli_task for #4 which should use the same serial port. Using a macro we exclude shell when mavlink is required, which is only the case when the olimex board is connected the the uart0, instead of the user.