boot: zephyr: main: fix log_process call after Zephyr change

The legacy logging subsystem was removed in below commit:
https://github.com/zephyrproject-rtos/zephyr/commit/c5f2cde



This commit maintains compatibility with Zephyr upstream before
and after the change by using the now removed CONFIG_LOG1 and
CONFIG_LOG2 defines.

Signed-off-by: Martin Jäger <martin@libre.solar>