Extend the platform logging config values

Extend the platform logging config values

  `plog` is a config value which controls whether platform logging is enabled,
  i.e. whether logs are even sent to the respective buffers. This is a global
  setting that affects all libdlog users indiscriminately but is very fast,
  unlike filters which have fine-grained control over who gets to log and how
  much but involve some processing.

 * `enable_X` can now be set to control this per buffer, for example to only
   control the `main` buffer, use `enable_main`. This overrides the global
   default from the `plog` value.

 * this can be set dynamically in the runtime config file, similar to filters.

 * the generic `plog` config value still works as a default for the three
   original buffers (`main`, `system`, `radio`) but can be overridden by
   the per-buffer settings. The `apps` buffer does not obey this setting
   and can only be controlled per-buffer. This is for backward compatibility.

Change-Id: I1c0facfe54e5aebeb28a54c808b93ddd1cff5125
Signed-off-by: Michal Bloch <m.bloch@samsung.com>