[Cryptech Tech] Revised keystore API and keystore flash "filesystem"

[Rob Austein]

Preliminary version of revised keystore API and flash management code
committed and pushed to branch ksng in sw/{libhal,stm32,pkcs11}
repositories.  Still needs work before it'll be ready to consider for
merging into the master branch, but the basic mechanism seems to work.
Not yet heavily tested.

NB: Keys and PINs saved to flash with the old keystore flash code will
not be preserved if you try this code (in theory they'll be the last
things overwritten, but the wear-leveling code will get around to
overwriting them eventually).  Sorry, it's a development platform,
adding a big chunk of backwards-compatibility code (read: seldom-used
code path, larger attack surface) seemed like a bad idea.  Can still
add backwards-compatibility if folks strongly disagree, of course.

Next steps:

* Switching from erasing entire flash sectors to erasing individual
  flash subsectors (in theory this is a trivial change, same C code
  should work, just a different opcode);

* Adding support for key objects larger than one flash subsector;

* Adding general attribute storage to key objects so we can start
  phasing out the current SQLite3 database used by the PKCS #11 code.