Kconfig: Include Kconfig.zephyr last in sample Kconfig files

This will make adding properties to symbols in the base Zephyr Kconfig
files work the same as before.

I didn't actually spot any such cases, so this is just to play it safe.
It also makes the sample Kconfig symbols appear at the top in the
menuconfig interface, which might be nice.

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