This adds the ability to control barebox over serial lines. The regular
console is designed for human input and is unsuitable for controlling
barebox from scripts since characters can be lost on both ends, the data
stream contains escape sequences and the prompt cannot be easily matched
upon.
This approach is based on the RATP protocol. RATP packages start with a
binary 0x01 which does not occur in normal console data. Whenever a
0x01 character is detected in the console barebox goes into RATP mode.
The RATP packets contain a simple structure with a command/respone
type and data for that type. Currently defined types are:

BB_RATP_TYPE_COMMAND (host->barebox):
	Execute a command in the shell
BB_RATP_TYPE_COMMAND_RETURN (barebox->host)
	Sends return value of the command back to the host, also means
	barebox is ready for the next command
BB_RATP_TYPE_CONSOLEMSG (barebox->host)
	Console message from barebox

Planned but not yet implemented are:

BB_RATP_TYPE_PING (host->barebox)
BB_RATP_TYPE_PONG (barebox->host)
	For testing purposes
BB_RATP_TYPE_GETENV (host->barebox)
BB_RATP_TYPE_GETENV_RETURN (barebox->host)
	Get values of environment variables

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Andrey Smirnov <andrew.smirnov@gmail.com>