bootutil: Add support for devices without erase and reduced erases

The commit adds two MCUboot configuration options:
  - MCUBOOT_SUPPORT_DEV_WITHOUT_ERASE
  - MCUBOOT_SUPPORT_DEV_WITH_ERASE
  - MCUBOOT_MINIMAL_SCRAMBLE

The first one should be enabled to support devices that do not require erase.
When such devices are used in system then MCUboot will avoid erasing such
device, which is not needed by hardware, and will just write data to it.
This allows to both improve device lifetime and reduce time of operations
like swap.

The second option is just bringing a configuration option for already existing
support for deviceses with erase.

The third option allows to reduce amount of removed data. When enabled,
MCUboot will remove enough of data, depending on the purpose of the removal,
to just fulfill the purpose; for example if removal of data is done to
make image unrecognizable for MCUboot, with this option, it will only
remove header.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>