subsys/settings: stream-style encoding and decoding to/from storage

This patch reworks routines used to store and read the settings data.
Provide stream-style encoding and decoding to/from flash, so the the
API only requires a pointer to binary data, and the settings
implementation takes care of encoding/decoding to/from base64 and
writing/reading to/from flash on the fly. This would eliminate the
need of a separate base64 value buffer on the application-side, thereby
further contributing to the stack footprint reduction.

Above changes allows to remove:
  256-byte value length limitation.
  removing enum settings_type usage so all settings data are treated
  now as a byte array (i.e. what's previously SETTINGS_BYTES)

Introduced routine settings_val_read_cb for read and decode the
settings data from storage inside h_set handler implementations.
h_set settings handler now provide persistent value's context
used along with read routine instead of immediately value.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>