NUCLEO_F746ZG build fails

[MarceloSalazar]

Tested https://github.com/mbedmicro/mbed/releases/tag/mbed-os-5.1.0-rc3
GCC, Windows

Configuration: Ethernet, 6LoWPAN

Build fails because SW2 and SW3 are not mapped to this target:

[Error] main.cpp@72,24: 'SW2' was not declared in this scope
[Error] main.cpp@73,26: 'SW3' was not declared in this scope
[ERROR] .\main.cpp:72:24: error: 'SW2' was not declared in this scope
 InterruptIn obs_button(SW2);
                        ^
.\main.cpp:73:26: error: 'SW3' was not declared in this scope
 InterruptIn unreg_button(SW3);
                          ^

We need to create a config mapping for the relevant targets using this example.

[ciarmcom]

ARM Internal Ref: IOTCLT-876

[rhenson]

Resolved through #57... may be.

@yogpan01 - please could you confirm.