scripts: kconfig: Extend the assignment-failed warning

 - Tailor the warning when the symbol has no prompt, explaining how
   promptless symbols get values. Add some anti-select propaganda too.

 - Reference the 'Setting configuration values' in the Board Porting
   Guide. It explains Kconfig.defconfig files.

Fixes: #8388

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