bootutil: add swap without scratch strategy

This implements a swap upgrade that does not use a scratch area. It
works by first moving all sectors in the primary slot up one position,
and then looping on moving sector of index X of the secondary slot to
index X of the primary slot, followed by moving sector X+1 of the
primary slot to X on the secondary slot, for each sector X.

The idea behind this implementation was initially suggested by Jehudi
Maes (@Laczen) and implemented on his own bootloader (ZEPboot).

Signed-off-by: Fabio Utzig <utzig@apache.org>