board: remove atstk1005_defconfig

Neither the kernel nor U-Boot have support for a 1005 board, so let's
get rid of this board configuration, the corresponding target
skeleton, kernel and busybox configuration and device table.

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