cmake: warn user if CPPFLAGS is set as environment setting

Zephyr doesn't use CFLAGS, CXXFLAGS, and CPPFLAGS if they are defined
in environment.

In case user defines CFLAGS or CXXFLAGS in environment, then a warning
message is printed, however CPPFLAGS does not raise a warning.

CPPFLAGS is updated to follow the principle of CFLAGS and CXXFLAGS.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>