subsys: logging: Fix for no CONFIG_LOG_PROCESS_THREAD

When CONFIG_LOG_PROCESS_THREAD was not set the logger
was not initialized.
Register a POST_KERNEL init for that case.

Also added an assert in the logger thread in case there
is no backends, instead of having that thread spinning
forever.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>