Some users report that current snapshot producies non-bootable images.
Stock uboot can boot images if the kernel is smaller than 2MB.
Set maximum kernel size and disable image building for this board.
Ref: https://forum.openwrt.org/t/astoria-arv7519rw22-bootloops-after-upgrade/89843
Signed-off-by: Aleksander Jan Bajkowski <A.Bajkowski@stud.elka.pw.edu.pl>
(cherry picked from commit c027dbac5a5222e6be4706a7a6a0effefce8ae76)