buildroot

board: update and minify at91sam9263ek_defconfig

Use modern kernel and U-Boot versions, and get rid of the now useless
kernel configuration file since we use the kernel defconfig file
instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>