hw/riscv: Allow direct start of kernel for MPFS

hw/riscv: Allow direct start of kernel for MPFS

Further customize the -bios and -kernel options behaviour for the
microchip-icicle-kit machine.  If "-bios none -kernel filename" is
specified, then do not load a firmware and instead only load and start
the kernel image.

For test runs, use an approach similar to
riscv_find_and_load_firmware().

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250319061342.26435-5-sebastian.huber@embedded-brains.de>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>