utils: Add cstr_token() function

utils: Add cstr_token() function

Add an auxiliary cstr_token() function to get a token from a const char
string. The function usage syntax is similar to str_token(), but unlike
str_token() the function doesn't modify the buffer of the string. Change
str_token() function implementation to use cstr_token().

Signed-off-by: Max Stepanov <Max.Stepanov@intel.com>
Reviewed-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>