fs: Kconfig: Clean up representation of mutually exclusive options

The flash and RAM disk options are mutually exclusive (evident e.g. in
both defining FS_VOLUME_SIZE and FS_BLOCK_SIZE variables), so they
should really be inside a choice-endchoice block. The duplicated
options can also taken care of with a single definition using
conditional default values.

Change-Id: I87733f0c342166a9e03aa0e1f34390d91107b137
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>