zephyr/boot_serial_extension: us BOOT_LOG instead of LOG_

Use MCUboot logging macros instead of zephyr-rtos native.
This fixes build failure on MCUBOOT_LOG_MODULE_DECLARE() when
LOG is disabled.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
(cherry picked from commit d16598f5)