pb-config: Move config storage modules to "platform" modules in discover code

pb-config: Move config storage modules to "platform" modules in discover code

There's no need to include the config storage code in lib/ as only the
discover server should be using it.

This change moves the config-storage code to discover/, with the
platform-specific parts moved to a 'struct platform'. Each platform has
a probe function, which is called during init. The first probe function
to return a platform is used.

At present we only have the one platform, but it's now non-intrusive to
add others.

We keep an array of platform pointers in a separate ("platforms")
section, to allow the test module to drop-in its own test "platform".

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>