configs/roseapplepi_defconfig: use gcc 7.x

The old 3.10.x based vendor kernel does not build correctly with gcc 8.x.

While there is basic s500 support in the mainline kernel, there is not yet a
mmc driver so it isn't quite a replacement yet.

Stick to the vender kernel for now and revert back to gcc 7.x, hopefully
mainline support will be more complete once gcc 7.x gets dropped.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>