boot:zephyr: CONFIG_LOG_MINIMAL converted to CONFIG_LOG_MODE_MINIMAL

CONFIG_LOG_MINIMAL was replaced by CONFIG_LOG_MODE_MINIMAL since
https://github.com/zephyrproject-rtos/zephyr/pull/31404

 PR was
merged.
CONFIG_LOG_MINIMAL is still emitted as legacy property, but
can't be assigned so prj.conf was fixed.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>