kconfig: enforce CONFIG_LOCALVERSION=flavor

The CONFIG_LOCALVERSION option /has/ to be equal to the package flavor in the kconfig as mkinitfs expects this to be the case, otherwise pmbootstrap install will fail with strange errors, add a check to enforce this.

This has been a bit of a point of contention for a while, I've personally hit this issue more times that I'd like to admit, lets prevent kernel packages being merged that don't have the correct LOCALVERSION set.

Initially I have this set to always check, but I suspect there will be exceptions to this. Due to the nature of the issues here I think it would be best to have this check enabled by default, with an option to disable it for kernels which shouldn't have it.