This matches the default EEPROM size of the underlying FlashStorage library, and
is substantially bigger than AVR-based keyboards, yet not _too_ big.
For various reasons, we're mirroring EEPROM into RAM 1:1, so we're constrained
by the size of that, too. That makes larger storage sizes undesirable at this
time. On top of this limitation, larger storage sizes also pose backup & restore
speed issues with Chrysalis, so lets settle for 16k, which is still very big,
all things considered, but not big enough to be a problem.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>