cmake: kconfig: Run Kconfig on every reconfiguration

Kconfiglib does not support merging fragments without parsing the
entire Kconfig "database". For Zephyr this means we no longer get a
performance gain from splitting up the fragment-merging and the
.config generation.

This patch removes the seperated merge step and it's associated
caching mechanism. Now we do a full Kconfig execution on every
reconfiguration.

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