kconfig: Change the format of KERNELVERSION

Change the format of KERNELVERSION as it exists in Kconfig to match
it's format elsewhere. Specifically, change from a string format
"1.10.3" to a numerical format.

This format change is a backwards-incompatible change. Any out-of-tree
Kconfig sources that were accessing KERNELVERSION with
$(KERNELVERSION) will need to be ported.

This change is to fix a bug. KERNELVERSION should have always been in
a numerical format. It should have been in a numerical format because;

KERNELVERSION is in a numerical format as a CMake variable and as a C
macro.

The purpose of having KERNELVERSION available in Kconfig is to have
configuration depend on the version, but this is only possible if it
exists in a numerical format[0].

[0] A string comparison of two semver strings does not always result
in a correct version comparison.

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