tools: kwboot: Use separate thread for sending boot message pattern

tools: kwboot: Use separate thread for sending boot message pattern

After BootROM successfully detects boot message pattern on UART it waits
until host stop sending data on UART. For example Armada 385 BootROM
requires that host does not send anything on UART at least 24 ms. If host
is still sending something then BootROM waits (possibly infinitely).

BootROM successfully detects boot message pattern if it receives it in
small period of time after power on.

So to ensure that host put BootROM into UART boot mode, host must send
continuous stream of boot message pattern with a small gap (for A385 at
least 24 ms) after series of pattern. But this gap cannot be too often or
too long to ensure that it does not cover whole BootROM time window when it
is detecting for boot message pattern.

Therefore it is needed to do following steps in cycle without any delay:
1. send series of boot message pattern over UART
2. wait until kernel transmit all data
3. sleep small period of time

At the same time, host needs to monitor input queue, data received on the
UART and checking if it contains NAK byte by which BootROM informs that
xmodem transfer is ready.

But it is not possible to wait until kernel transmit all data on UART and
at the same time in the one process to also wait for input data. This is
limitation of POSIX tty API and also by linux kernel that it does not
provide asynchronous function for waiting until all data are transmitted.
There is only synchronous variant tcdrain().

So to correctly implement this handshake on systems with linux kernel, it
is needed to use tcdrain() in separate thread.

Implement sending of boot message pattern in one thread and reading of
reply in the main thread. Use pthread library for threads.

This change makes UART booting on Armada 385 more reliable. It is possible
to start kwboot and power on board after minute and kwboot correctly put
board into UART boot mode.

Old implementation without separate thread has an issue that it read just
one byte from UART input queue and then it send 128 message pattern to the
output queue. If some noise was on UART then kwboot was not able to read
BootROM response as its input queue was just overflowed and kwboot was
sending more data than receiving.

This change basically fixed above issue too.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>