riscv: qemu: support booting Linux

Support booting Linux (as payload of BBL) from FIT images. For this, the
default CONFIG_SYS_BOOTM_LEN is increased to 16 MB, and the environment
variables fdt_high and initrd_high are set to mark the device tree and
initrd as in-place.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/qemu-riscv32_defconfig b/configs/qemu-riscv32_defconfig
index 72c54a3..b556443 100644
--- a/configs/qemu-riscv32_defconfig
+++ b/configs/qemu-riscv32_defconfig
@@ -1,6 +1,7 @@
 CONFIG_RISCV=y
 CONFIG_TARGET_QEMU_VIRT=y
 CONFIG_NR_DRAM_BANKS=1
+CONFIG_FIT=y
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_DISPLAY_BOARDINFO=y
 CONFIG_OF_PRIOR_STAGE=y