zephyr: allow recovery over usb cdc-acm with logs enabled

Modified serial_adapter so log are allowed when using USB
CDC ACM serial port emulation.

Added dedicated thread for log processing of the highest application
priority. This allows to transmit all logs without adding k_sleep
anywhere else int the code.

Introduced boot log thread is simpler than the default log threat
which decreases flash footprint by a few dozen bytes.
Added configuration for nrf52840_pca10056 which shows how
to enable looging along with USB - among other, thread log
processing is required.

build command (form zephyr-project root directory)
west build -d build/mcuboot/nrf52840_pca10056 -b nrf52840_pca10056
./bootloader/mcuboot/boot/zephyr/
 -- -DDTC_OVERLAY_FILE=./boards/nrf52840_pca10056_big.overlay
-DOVERLAY_CONFIG=./usb_cdc_acm_log_recovery.conf

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>