boards: beagleconnect_freedom: Enable swap using scratch

- The fs layout for MCUboot on bcf has the following requirements:
  - slot0 and slot1 are on different flash (say A and B).
  - flash A and B have different sector sizes (8K and 4K).
  - slot1 should be >= slot0. This is due to the fact that slot0 is in
    the soc flash. Hence want to have as much usable space as possible.

- Taking all of the above into consideration, swap using scratch seems
  to be the appropriate algorithm for bcf.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>