logging: syst: fix logging v2 with strings as arguments

The packaged string coming from logging subsys requires some
additional processing if there are string arguments.
These strings are actually embedded inside the package so
the string pointers inside the argument list must be
replaced with pointers to strings inside the package.
Without this extra step of processing, MIPI Sys-T's printf
function would process these arguments directly which may be
NULL pointers or invalid one pointing to somewhere.

This utilizes the new cbpprintf_external() for the processing
before feeding data to the MIPI library.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>