scripts: kconfig: Put a blank line before warnings and errors

Since the warnings generated by this script are so spammy, it can be
difficult to tell where they start and end. Put a blank line before each
one to make it clearer.

Also put blank lines around the non-whitelisted-warning error.

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