kconfiglib/menuconfig/kconfig.py: Various improvements

Update Kconfiglib and menuconfig to upstream revision d3866958c7685, to
add various improvements:

 - Support HOME and END in the jump-to dialog in the menuconfig. END can
   be handy as choices, menus, and comments appear at the end.

 - Add more fine-grained warning controls for multiple assignments to a
   symbol in configuration files. Use it to simplify kconfig.py a bit.

   Clean up kconfig.py a bit in other ways too, e.g. by removing unused
   imports.

 - Improve Kconfig parsing performance slightly

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>