boards: defconfig: Consistently quote string defaults

In preparation for introducing a warning.

Unquoted string defaults work through a quirk of Kconfig (undefined
symbols get their name as their string value), but look confusing. It's
done inconsistently now too.

Suggested by Kumar Gala.

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