cmake: improve warning deprecation message

The current deprecation warning could give the impression that
CROSS_COMPILE is deprecated, which is not the case.

Instead it is the behavior of defaulting to `cross-compile` when
setting CROSS_COMPILE in environment without setting
ZEPHYR_TOOLCHAIN_VARIANT that is deprecated.
Not the use of `CROSS_COMPILE` setting itself.

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