kconfig: Drop support for specifying CROSS_COMPILE through Kconfig

When we move DT infront of Kconfig we are going to need access to a C
toolchain before Kconfig is evaluated. This means it will not be
possible to specify the toolchain used through Kconfig.

To deal with this we ...

Drop support for specifying CROSS_COMPILE through Kconfig. Still
available is the ability to specify CROSS_COMPILE through the
environment or through a CMake variable.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>