Raise the compilation warning level and rename the default build configuration

[gregbreen]

By default, the warning level is low. Turn it up, fix easy warnings, disable a handful that are too hard to fix in SiLabs' code.

I also don't lile the default build configuration name of "GNU ARM v7.2.1 - Default". I understand where it comes from, but I prefer the decades old tried and true Debug and Release.

Create a Debug build configuration, with optimization O0. Also define DEBUG macro constant for that build configuration.

[gregbreen]

Like the Thunderboard React, turning on Wextra produced some warnings in Silicon Labs code. Changes here: https://github.com/gregbreen/uncannier-thunderboard-sense2/pull/3/files

Selecting disabled Wunused-parameter because there were too many of them.

-Wconversion and -pendatic also produced too many warnings to enable those.

Note that advertisingIteration() in radio_ble.c already has a DEBUG guard around the EM4 shutdown mode. So the Debug build configuration will never enter EM4 shutdown.