nuttx: add support for swap without scratch area

Definition of MCUBOOT_SWAP_USING_MOVE in case swap without scratch area
is configured in NuttX was missing from mcuboot_config.h file.

Also necessary function flash_area_sector_from_off() is defined and
declared in order to support swap without scratch.

Signed-off-by: Michal Lenc <michallenc@seznam.cz>