build: support building with subprojects

Enable everything to build outside of an OE-SDK by leveraging
subproject wrap files.  Needed to update C++ to C++20 as well
in order to allow phosphor-logging to compile.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I43f5fcd53ef3ac024d7b5ce50fd035903d08bab0
7 files changed