README.kconfig: document backward compatibility "make *_config"

Commit 3ff291f371fa9858426774f3732924bacb61ed1c
(kconfig: convert Kconfig helper script into a shell script)
restored "<board>_config" target for backward compatibility.
It should be documented.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>

doc/README.kconfig

@@ -114,6 +114,13 @@ See below for how each configuration target works in U-Boot:
   coalesced together with "<condition:>" prefix for each line as shown above.
   This file can be used as an input of "defconfig" target.
 
+- <board>_config
+
+  This does not exist in Linux's Kconfig.
+  Prior to Kconfig, in U-Boot, "make <board>_config" was used for the
+  configuration.  It is still supported for backward compatibility and
+  its behavior is the same as "make <board>_defconfig".
+
 
 Migration steps to Kconfig
 --------------------------

scripts/multiconfig.sh

@@ -248,6 +248,7 @@ case $target in
 *_defconfig)
 	do_board_defconfig $target;;
 *_config)
+	# backward compatibility
 	do_board_defconfig ${target%_config}_defconfig;;
 silentoldconfig)
 	do_silentoldconfig;;