linux: fix uImage location on AVR32

On most architectures, the kernel image can be found in
arch/<ARCH>/boot, but on AVR32, it's in arch/<ARCH>/boot/images.

Issue initially reported by Joachim Pihl
<joachim.pihl@sensordevelopments.com>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>