sys/util: fix macro support when building with C++

true and false in a C environment are macros that are defined to 1 and
0 respectively.  In C++ they are keywords, and as such cause failures
when macro tests require they expand to a 1 or 0 token.  Use the
actual token values rather than the macro names when defining the
macros.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>