package/vnstat: New package

[Thomas:
  - Rename the VNSTAT_INSTALL_VNSTATI to VNSTAT_INSTALL_VNSTATI_CMDS,
    and use 'define ... endef'.
  - Use full paths for the destinations when using $(INSTALL)]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>