When you set BR2_PACKAGE_LINUX_USE_DEFCONFIG

to configure linux using the

make <board>_defconfig

You have to supply the $(BOARD_NAME) variable.
Add this info in the help text for
target/linux/Makefile.in.advanced

----------------

When you do not set BR2_PACKAGE_LINUX_USE_XCONFIG
to configure linux using "$make <XXXX>config
The choices "$make menuconfig" or "$make xconfig"
should not be visible.
Add dependency

target/linux/Config.in.advanced

@@ -437,6 +437,8 @@ config BR2_PACKAGE_LINUX_USE_DEFCONFIG
 	depends on BR2_PACKAGE_LINUX
 	help
 	  Configure Linux by make <board>_defconfig
+	  You have to supply a BOARD_NAME, either by
+	  defining it in a BSP or by make BOARD_NAME=<board>
 
 config BR2_PACKAGE_LINUX_USE_XCONFIG
 	bool "Run make ARCH=$(ARCH) [xconfig|menuconfig] before build"
@@ -445,8 +447,6 @@ config BR2_PACKAGE_LINUX_USE_XCONFIG
 	  Configure Linux by make xconfig
 endchoice
 
-
-
 config BR2_PACKAGE_LINUX_KCONFIG
 	string ".config file"
 	depends on BR2_PACKAGE_LINUX_USE_KCONFIG
@@ -462,6 +462,7 @@ choice
 	prompt "Preferred config tool"
 	default BR2_MAKE_XCONFIG if BR2_TARGET_ATMEL
 	default BR2_MAKE_MENUCONFIG
+	depends on BR2_PACKAGE_LINUX_USE_XCONFIG
 	help
 	  Define the preferred tool to invoke if a configuration is invalid