Log_generic() can be used by external logger systems to map to zephyr
logger subsystem. Function has variable number of arguments. So far
logger assumed maximal number of arguments to avoid scanning
string to count number of arguments. In outcome, uninitialized memory
was treated as valid argument in a message (but discarded when actual
string formatting took place). Valgrind complained about that so
log_generic() has been extended to scan the string and count number
of arguments and build message with valid number of arguments.
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>