* Fix behaviour vith -j flag. $(MAKE) must be a separate command.
* Separate CFLAGS logic to standalone snippet
  * Variable PREPROCESS_ONLY for running only preprocessor
* Add something like SYS_LIBS and SYS_OBJ (or DEFAULT_LIBS...). These
   could be defined in board config (or arch config TODO) and will
   specify the names of libraries which are loaded by default. It will
   be like exception handlink, system_stub etc. If these are not
   needed, linker will not include them.
* Add SUBDIRS_OPT variable to Makefile.omk. It will descent to the
   specified directories only if the directory exists. In case of
   non-existing directory, no error is produced.