pandaboard: Fix defconfig build

Commit 75b7dc68 ("linux: default to 3.9, remove 3.8") made 3.9 the
default kernel version.

The omap kernel switched to multiplatform kernel during the 3.9 kernel
development cycle. Obviously, the uImage generation doesn't work anymore
with multiplatform kernels, since you have to provide the kernel load
address, resulting in a build error.

Lock down the kernel version to 3.8 to keep the old behaviour until
someone submits a patch to switch to multiplatform kernel.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>