making UART/USART handler weak to allow custom external handlers

[facchinm]

Moved from https://github.com/arduino/Arduino/pull/3023

ricardocrudo commented on 23 Apr 2015 I've opened a thread about this here: https://groups.google.com/a/arduino.cc/forum/#!topic/developers/FIjuI4z06tY

[odurc]

I'd like, one more time, to bring attention to this pull request. It's very simple to review and might be very useful for advanced users to create customized/optimized libraries as I'm doing at my company. On the other hand, merging it won't affect the regular functionality of the UART/USART.

[Wurstnase]

+1 Please also for the PIOx_Handlers.

[livello]

Need this patch too!!!

[bankian]

Please could someone give an update on the status of this request - it would be very useful!

[CLAassistant]

All committers have signed the CLA.