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>

Kconfig

@@ -12,12 +12,6 @@ config KCONFIG_OBJDIR
 	string
 	option env="KCONFIG_OBJDIR"
 
-config DEFCONFIG_LIST
-	string
-	depends on !SPL_BUILD
-	option defconfig_list
-	default "configs/sandbox_defconfig"
-
 menu "General setup"
 
 config SPL_BUILD