kconfig: remove DEFCONFIG_LIST

CONFIG_DEFCONFIG_LIST specifies the default defconfig.
It is used by "make savedefconfig" when .config is missing.
But that's it.  I could not find other useful cases.

As a side effect, CONFIG_DEFCONFIG_LIST="configs/sandbox_defconfig"
is contained in .config of every target board, which some people
think is odd.  So, let's remove it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>