This defconfig use a custom defconfig file located
in board/friendlyarm/nanopi-r1/uboot/nanopi_r1_defconfig.

BR2_TARGET_UBOOT_BOARD_DEFCONFIG is used to provide the name
of a in-tree defconfig. Since a custom defconfig is used,
this option is lost while loading the defconfig with:
make nanopi_r1_defconfig

This fixes the new defconfig check.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>