[IA64] move defconfig to arch/ia64/configs/

This patch moves the default ia64 defconfig to
arch/ia64/configs/generic_defconfig where it belongs and selects it as
the default defconfig through KBUILD_DEFCONFIG.

Signed-off-by: Adrian Bunk <adrian.bunk@movial.fi>
Signed-off-by: Tony Luck <tony.luck@intel.com>