buildroot

configs: remove old/broken Atmel configurations

The at91rm9200df_defconfig configuration is really old: it builds a
2.6.38 kernel with the old days kernel patches and a 2010.06
U-Boot. It hasn't been updated since a long time, so let's get rid of
it.

The at91sam9260dfc_defconfig, at91sam9261ek_defconfig and
at91sam9263ek_defconfig refer to kernel defconfigs that no longer
exists, so they fail to build.

Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>