bootloader for ATmega328 U

Error message "expected signature for ATmega328P is 1E 95 0F

use -F to override this check"

What does "-F" mean?

How do you tell the thing to override this code?

Or where could one find the appropriate code for a 328 U and how could it be inserted in the correct place?

You can pass "-F" as parameter to avrdude tool, but definitely you shouldn't do it. It doesn't help.
First of all, you should post whole log. Do not forget to use code tags.
Secondly, you have different MCU. ATmega328P and ATmega328 have different signature. Second one have 1F 95 14 so logically the error occurs. The solution in such case should be to create specific board for 328 (in boards.txt).