Move Serial example 3 to RawSerial

[yennster]

Example three in the Serial API doc attaches an interrupt to the Serial pc object, and inside the callback function getc() and putc() are used. As of the latest version of Mbed OS, both of these functions use mutex locks which are not allowed in ISR context, and create the following error at runtime:

++ MbedOS Error Info ++
Error Status: 0x80010133 Code: 307 Module: 1
Error Message: Mutex: 0x20000F1C, Not allowed in ISR context
Location: 0x1002B29
Error Value: 0x20000F1C
Current Thread: Id: 0x20002850 Entry: 0x10040C9 StackSize: 0x1000 StackMem: 0x20001810 SP: 0x2003FE50 
-- MbedOS Error Info --

In order to use getc() and putc() in ISR context with a serial interrupt, you need to use a RawSerial object.

If these changes get approved, I can either make a new example for RawSerial or update the existing example 3 for Serial here: https://os.mbed.com/teams/mbed_example/code/Serial_ex_3/

[geky]

I looks like this example is still using the Serial class?

[yennster]

@geky Yeah, I would update that example if these docs changes get approved

[yennster]

And change the repo name to "RawSerial_ex_2"

[AnotherButler]

@SenRamakri Are you happy for this to merge?

[yennster]

@SenRamakri do you have any comments?

[SenRamakri]

Looks good..

[yennster]

@AnotherButler this is ready, and I've updated the example in https://os.mbed.com/teams/mbed_example/code/RawSerial_ex_2/