mynewt: Add support for MCUBOOT_SINGLE_APPLICATION_SLOT

So far mynewt code required definition of second slot in bsp.
Even in cases when overwrite only option was used bsp had to
have secondary slot with 0 length or pointing to some
unreachable flash or flash area.

Now when BOOTUTIL_SINGLE_APPLICATION_SLOT is set to 1
there is not need for fake flash areas when mcuboot
provides way for upgrade like boot serial or other
supported ways (USB DFU or USB MSC)

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>