#109 Allow explicit break control on TX line

[Alex]

Currently, it is possible to forces the UART to send a BREAK sequence on TX line using string "brk" in the GPIO entry/exit sequence

There are two problems here:
- The duration cannot be controlled
- tcsendbreak() is used to generate the BREAK condition with the duration parameter hardcoded to 1. According to POSIX (https://pubs.opengroup.org/onlinepubs/009695399/functions/tcsendbreak.html) the actual duration when the parameter is non-zero is implementation-defined.

As an alternative, I think it is a good option to use the system calls and control the TX line manually using TIOCCBRK/TIOCSBRK.
This way, the duration can be controlled in the GPIO entry/exit sequence.
I add a patch to implement this new feature.

Example (usage):
stm32flash -i brk,,,-brk -w FILE /dev/ttyUSBX