console: Introduce console_drain() ...

Generalize high baud rate UART polling code found in lib/xmodem.c and
expose it as a generic function in console API.

The usecase for this -- besides X/Y-mode data transfers where the code
originated -- is command/reply type of serial exchages at high baud
rates (~1Mbaud) with payloads exceeding inernal UART FIFOs in
size. Such interactions are not uncommon in serdev device and this
patch is done in preparation for serdev support code that will follow.

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

xymodem.c: fix compiler warning ...

Fixes:
 lib/xymodem.c:558: warning: no previous prototype for ‘do_load_serial_xmodem’
 lib/xymodem.c:572: warning: no previous prototype for ‘do_load_serial_ymodem’
 lib/xymodem.c:586: warning: no previous prototype for ‘do_load_serial_ymodemg’

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

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>