Makefile: implement GNU Coding Standard for Makefiles

GNU coding standards notably specifies:
* install files with the $(DESTDIR) to the target system image
* install files with the $(prefix), not $(PREFIX)
* the default value of $(prefix) should be /usr/local

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

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Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>