riscv: qemu: clear kernel-start/-end in device tree as workaround for BBL

QEMU specifies the location of Linux (supplied with the -kernel
argument) in the device tree using the riscv,kernel-start and
riscv,kernel-end properties. We currently rely on the SBI implementation
of BBL to run Linux and therefore embed Linux as payload in BBL. This
causes an issue, because BBL detects the kernel properties in the device
tree and ignores the Linux payload as a result.
Work around this issue by clearing the kernel properties in the device
tree before booting Linux.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/board/emulation/qemu-riscv/Kconfig b/board/emulation/qemu-riscv/Kconfig
index be5839b..33ca253 100644
--- a/board/emulation/qemu-riscv/Kconfig
+++ b/board/emulation/qemu-riscv/Kconfig
@@ -30,5 +30,6 @@
 	imply CMD_EXT4
 	imply CMD_FAT
 	imply BOARD_LATE_INIT
+	imply OF_BOARD_SETUP
 
 endif