Makefile: be more compliant with GNU coding standards
Makefile: be more compliant with GNU coding standards

* https://www.gnu.org/prep/standards/html_node/Directory-Variables.html

Signed-off-by: Vladimir Oltean <6ef50fec496bca30294ed8eff4f157ee91f2dc55@nxp.com>

openil/sja1105-tool,openil/sja1105-tool