subsys/fs/littlefs: allow customization of file system configuration

There's desire to be able to customize parameters on a per-filesystem
basis, which means we need a way to override the Kconfig defaults which
are global.  This also means the littlefs data structure cannot own the
cache and lookahead buffers.

Switch to using a macro to define the littlefs data structure.  The
default version uses the Kconfig constants.  A custom one takes
arguments providing the most likely partition-specific parameters.
Finally the user is free to bypass the helper macros and set any
parameters desired, though validation is limited and only present when
CONFIG_DEBUG is enabled.

Extend the test suite with a performance module, which confirms that
these settings have an impact proportional to the log of changes to the
cache or IO sizes.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>