Using underscore prefixes is not allowed for include guards as all
defines starting with underscore are reserved. At the same time made
file ends and spacing and style of include guards consistent in all
headers.
The new style of include guards also includes the directory of the
include file relative to `src` directory to prevent potential future
issues where we might end up with a name clash across directories.
* DI layout is identified by attempting to read OUI from both layouts
* MSC address is passed to flashstub in r3
Retested EZR32LG230 (EZR Leopard Gecko M3)
Tested EFR32BG13P532F512GM32 (EFR Blue Gecko)
Achieves aims of PR #264 (I think) Thanks to @dholth and @ryankurte for inspiration
Fixes Issue #226