commands: change Y-Modem implementation ...

The current Y-Modem implementation has some limitations:
 - Y-Modem/G protocol is not supported
 - Multiple files (aka. batch) transfers are not supported
 - Transfer speed over fast lines (USB console) is slow
 - Code is not trivial to maintain (personnal opinion)

This implementation tries to address all these points by
introducing loady2 command.

The effects are :
 - transfer speed for Y-Modem over USB jumps from 2kBytes/s
   to 180kBytes/s
 - transfer speed for Y-Modem/G jumps to 200kBytes/s
 - multiple file transfers are possible

This command was tested on a USB console and UART 9600bps
serial line :
 - NAKs (and retransmissions) were tested for faulty
   serial lines
 - multiple file transfers were tested
 - Y-Modem, Y-Modem/G and X-Modem transfers were tested

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Tested-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>