riscv/boot: Fixup the RISC-V firmware warning

riscv/boot: Fixup the RISC-V firmware warning

Fix a typo in the warning message displayed to users, don't print the
message when running inside qtest and don't mention a specific QEMU
version for the deprecation.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>