ARC: MWDT: cleanup search path handling

Cleanup ARC MWDT search path handling:
 * Don't print warning if ARCMWDT_TOOLCHAIN_PATH is missing but
   METAWARE_ROOT is set. Treat it as valid case and do message
   with status level instead of warning.
 * Make rest of the error messages more understandable for users.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>