scripts: kconfig: Use unique_{defined_syms,choices} and clean up a bit
scripts: kconfig: Use unique_{defined_syms,choices} and clean up a bit

 - Use Kconfig.unique_{defined_syms,choices} to avoid redundant checks
   for symbols/choices defined in multiple locations. These were
   recently added to Kconfiglib.

 - Remove the comment about the alldefconfig starting state. It probably
   isn't useful here.

 - Print the messages about loading configuration files just before they
   are actually loaded. That looks less confusing if one of them fails
   to load.

 - Line-wrap the error message about non-whitelisted warnings.

 - Misc. other minor code cleanup.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,nashif/zephyr,punitvara/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,kraj/zephyr,Vudentz/zephyr,galak/zephyr