Serial.flush modification

[dsv19]

Added a few defines to extract width, parity and stop from uart mode in uart.c as suggested in the comments in #5094 and modified flush in HardwareSerial.cpp file.

[d-a-v]

CI fails: uart.h needs to be committed too.
Should we count the implicit start bit too (+1 in the result, 12 instead of 11) ?
Is it necessary to wait (to change to 11/12) when the returned value is 0 ?

[dsv19]

I'm not sure about the implicit start bit but it is not necessary to wait when the returned value is 0 since the HardwareSerial::flush function already handles a situation when uart is null.

[d-a-v]

Start bit is always there, so IMHO 1 should be added to the tmp calculus.
Btw could you rename it for the readers, like bit_length or anything more explicit.
After a second read, you don't even need this variable, the value can be directly returned,
then there would be a comment to describe what's returned.

[d-a-v]

Thanks!

[earlephilhower]

Seems like an improvement, and makes good sense.