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

.

Suggested-by: Kirill Kapranov <kkapra@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>