Refactor board configurations

[danngreen]

Goal:

1. Allow selection of board with build system. This will allow us to build for multiple boards without changing any source code (currently you have to change a using namespace Board =...

2. Allow specifying custom board config file, which may be outside of mp1-boot source dir. For instance, including mp1-boot in another project

3. Allowing adding new options to board config without forcing all board config files to be updated.

Proposal:

Goals 1 and 2 can be done by building with make BOARD_CONF=xxx, where xxx can be a known board (DK2 or OSD32), or a path to a custom board_conf.hh file. This has been roughly implemented in the ssbl-option branch (still needs work).

Goal 3 can be addressed by changing namespace Board to struct Board, which inherits from a struct of constexpr members with default values. A new field can be added to the default struct, and all boards will inherit the default. Thus creating new fields won't break existing board configs.

With Board as a class type, it can be passed as a template parameter, which might be useful for checks like if constexpr (requires {typename Board::SomeThing})