u-boot: do not default to $(BOARD_NAME) for the board name

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/u-boot/Config.in

@@ -6,7 +6,6 @@ menuconfig BR2_TARGET_UBOOT
 if BR2_TARGET_UBOOT
 config BR2_TARGET_UBOOT_BOARDNAME
 	string "board name"
-	default "$(BOARD_NAME)"
 	help
 	  One of U-Boot supported boards to be built.
 	  This will be suffixed with _config to meet U-Boot standard naming.