kconfig: Avoid potential issue parsing generated_dts_board.conf

Splitting a string like 'foo="bar=baz"' on '=' will give ['foo', '"bar',
'baz"'] instead of the intended ['foo', '"bar=baz"']. split() with
maxsplit=1 to avoid potential issues.

Not seen in practice. Just some future safety.

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