logging: Add basic userspace support

This commit adds basic userspace support to the logging subsystem.
With this change, the following API could be called from user mode:
- LOG_*()
- LOG_INST_*(),
- LOG_HEXDUMP_*(),
- LOG_HEXDUMP_INST_*(),
- LOG_PANIC(), LOG_PROCESS(),
- log_printk(), log_generic(), log_buffrered_cnt(),
- log_filter_set(NULL, ...)

With userspace disabled, the logger behavior and performance
is not affected. With userspace enabled, the calls from kernel
space have an additional overhead introduced by _is_user_context().

The logger behavior changes when it is called from the user context.
All strings logged using LOG_*() and LOG_INST_*() API from userspace
are rendered in place for security reasons and then placed in
log_strdup() memory pool, which should be large enough to hold bursts
of log messages.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>