riscv: qemu: detect and boot the kernel passed by QEMU

QEMU embeds the location of the kernel image in the device tree. Store
this address in the environment as variable kernel_start. It is used in
the board-local distro boot command QEMU to boot the kernel with the
U-Boot device tree. The QEMU boot command is added as the first boot
target device.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alexander Graf <agraf@suse.de>