grub2: modify kernel location to /boot/zImage

This is the location where buildroot install the image if requested,
so this should be the default search path for the bootloader.

Signed-off-by: Alvaro G. M <alvaro.gamez@hazent.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>