Kernel version 4.14.15 is not available anymore and 4.17.3
gets built anyway. So let's set 4.17.3 as preferred version
for the Bananapi M1 + to avoid the build complaining 4.14.15
kernel recipe is missing from the layers.
Changelog-entry: Set PREFERRED_VERSION linux-mainline to 4.17.3 for bananapi-m1-plus
Signed-off-by: Florin Sarbu <florin@resin.io>
