logging: Add support to backends to switch log format at runtime.

Adding mechanism to all the backends to switch the logging formats
at runtime while leveraging the function pointer table based upon
the Kconfigs which also cleans up the logging backend design.
Also demonstrate the working API with the changes
to syst sample. Clean up Kconfig for backends and add a standard
template to generate the Kconfigs for logging formats to be used
by all backends.

Signed-off-by: Aastha Grover <aastha.grover@intel.com>