[fix] make nvm.clean - don't raise error when NVM is not installed

Issue was::

    $ LANG=C make nvm.clean
    INFO:  NVM is not installed
    make: *** [Makefile:99: nvm.clean] Error 42

Now::

    $ LANG=C make nvm.clean
    CLEAN     [NVM] not installed

BTW: change info_msg to build_msg

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>