Feature: Add non-blocking serial break/unbreak functions

[mray190]

Description

Current serial implementation has a send_break() command which sends a break command on the UART for a fixed amount of time. Fixed amount of time (in uS) is 18000000 / baud_rate. Additionally, the command is blocking.

This PR adds functions to the serial class to allow users to send a break in a non-blocking fashion, as well as for a user-specified amount of time.

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[x] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[mray190]

Can ignore the 2 referenced PRs/commits - Pasted in this PR link for future reference and have nothing to do with this PR into mbed-os

[kjbracey]

Seems reasonable to me - obviously you need to know what you're doing, and not mix this with any output calls.

Would need more work to make it work with UARTSerial, but that could be an exercise for later. (Doesn't currently support send_break either.)

[geky]

Looks good 👍

[kjbracey]

Ah, sorry, forgot to push my one comment to this. I think this is my only issue.

[mray190]

@kjbracey-arm Added locks and unlocks surrounding each uart break call

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3652
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8671/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3259
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8671/

[mbed-ci]

Test : SUCCESS

Build number : 3432
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8671/3432