mynewt: Add single_loader support

There is existing functionality for Zephyr where mcuboot works
with single slot (no swap) and image can be updated via boot_serial.

To have same functionality in mynewet single_loader.c file is copied
from zephyr tree and 2 pkg.yml files are modified to utilize new
file when BOOTUTIL_SINGLE_APPLICATION_SLOT is defined

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