Kconfiglib: Make header symbol order match .config files again

Commit f6bf8977 ("kconfiglib: Add preprocessor and two warnings")
accidentally made the ordering of #define's in generated header files
random (dependent on Python's set() implementation). Minimal
configuration files (which can be saved from the menuconfig) were
affected too.

The intent is for the order to match .config files, which in turn match
symbol definition order. This doesn't affect behavior in any way, but
makes things more readable (and is handier if files are checked in).

Update Kconfiglib to upstream revision a3252f620fad9 to make the
ordering match again. Tests have been added upstream.

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