Adds user configuration to the BDK.

Currently the user can control 3 values:
metrics_opt_in, bsp_dir, and platform_cache.
These can be controlled via `bdk config set`,
and checked via `bdk config check`.

Also fixes some minor caching logic where 'None'
value couldn't be cached.

BUG: https://b/28088083, https://b/27435023, https://b/28198008
TEST: unit tests pass
TEST: bdk config set/check tested
TEST: bdk environment setup, build platform, build image tested

Change-Id: Ifab049db71c7c124f173ffd34b4fb7824d4b4704
27 files changed