menuconfig: Add two small error reporting and UI improvements

Update menuconfig and Kconfiglib to upstream revision 68426efeae6aa, to
add two minor menuconfig improvements and a small bugfix:

 - Expected errors, like syntax errors in Kconfig files, no longer
   generate a Python backtrace. Just the error message is shown.

 - Entering a choice now places the cursor on the selected symbol, if
   any.

 - Having two consecutive empty 'if's (which could also appear e.g. due
   to osource) crashed the menuconfig when entering show-all mode, due
   to the 'if' removal logic in Kconfiglib failing to remove one of
   them. All configurations of 'if's are now correctly removed.

   This error was found while reading the code.

Some minor optimizations and some internal cleanup is included as well.

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