kirkwood: increase kernel size of Linksyses

kirkwood: increase kernel size of Linksyses

Linksyses Audi EA3500 and Viper E4200/E4500 have too small kernel
partition size when kernel 5.10 is used. This patch change kernel
partition to maximum size allowed by u-boot.

Kernel size is overlapping rootfs now, like mvebu Linksyses and stock
partition table. It fix  back to stock via sysupgrade, which was broken
since 18.06.

Fixes: 9808b9ae02 ("kirkwood: switch to kernel 4.9")
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>