doc: fix documentation of out-of-tree build

Correct environment variable for output directory is KBUILD_OUTPUT.
Signed-off-by: NTimo Ketola <timo@exertus.fi>

README

@@ -5012,14 +5012,14 @@ this behavior and build U-Boot to some external directory:
 	make O=/tmp/build NAME_defconfig
 	make O=/tmp/build all
 
-2. Set environment variable BUILD_DIR to point to the desired location:
+2. Set environment variable KBUILD_OUTPUT to point to the desired location:
 
-	export BUILD_DIR=/tmp/build
+	export KBUILD_OUTPUT=/tmp/build
 	make distclean
 	make NAME_defconfig
 	make all
 
-Note that the command line "O=" setting overrides the BUILD_DIR environment
+Note that the command line "O=" setting overrides the KBUILD_OUTPUT environment
 variable.
 
 

doc/README.kwbimage

@@ -30,10 +30,10 @@ kwbimage support available with mkimage utility will generate kirkwood boot
 image that can be flashed on the board NAND/SPI flash.  The make target
 which uses mkimage to produce such an image is "u-boot.kwb".  For example:
 
-  export BUILD_DIR=/tmp/build
+  export KBUILD_OUTPUT=/tmp/build
   make distclean
   make yourboard_config
-  make $BUILD_DIR/u-boot.kwb
+  make u-boot.kwb
 
 
 Board specific configuration file specifications: