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

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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