log: normalize log target condition check

THis doesn't change the condition's logic at all, but is an attempt to
make things a bit more readable: instead of checking log_target !=
LOG_TARGET_AUTO let's actually list the targets where we want to
consider journal/syslog/kmsg, to make things a bit less confusing. After
all the message here is not to avoid them if LOG_TARGET_AUTO is set, but
to definitely do them in the other cases.

(cherry picked from commit ef9bddb7)