pandaboard: fix defconf build with recent kernels

The patch "pandaboard: Fix defconfig build" locked the
default pandaboard kernel to 3.8.11. This is inconvenient,
since the boot process has changed since kernel > 3.8.

Update the default kernel to 3.12.2 and fix the boot image
configuration, but remain compatible with legacy U-Boot
versions by using appended flattened device tree uImage.

This change *should* keep future kernels compatible with a
fixed U-Boot version.

Note the default device tree is now for the pandaboard-ES,
so users of other board revisions will need to set the
correct DTS_NAME.

[Peter: use custom kernel headers version (3.12.2)]
Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>