build: image: Fix off-by-one in DTC kernel version checks

build: image: Fix off-by-one in DTC kernel version checks

It was reported to me on IRC today, that my change causes issues with
kernel versions between 4.14 and 4.19.

It's because I've wrongly used `git describe` in order to get kernel
version where we should disable noisy DTC checks, but I should've used
`git tag --contains` instead.

Fixes: cbbef976e2b ("build: dtc: Disable noisy warnings by default")
Signed-off-by: Petr Štetiar <ynezz@true.cz>