Allow overriding the ESP8266 timeouts

[janjongboom]

By wrapping these in an #ifndef these can be overridden from user space (e.g. mbed_app.json). For a justification, see https://stackoverflow.com/questions/45328298/mbed-socket-connection-takes-a-long-time.

@geky @sarahmarshy

[sarahmarshy]

@janjongboom I don't see how this is related to ESP8266. The person in that thread is using ethernet...

[janjongboom]

@sarahmarshy Ah, wow. Brain must have melted. Regardless, I think it's good to add these ifndef's.

[geky]

This makes sense, we should probably also create an mbed_lib.json, but we can merge this for now