hw/riscv/boot.c: Introduce riscv_find_firmware()

hw/riscv/boot.c: Introduce riscv_find_firmware()

Rename previous riscv_find_firmware() to riscv_find_bios(), and
introduce a new riscv_find_firmware() to implement the first half
part of the work done in riscv_find_and_load_firmware().

This new API is helpful for machine that wants to know the final
chosen firmware file name but does not want to load it.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221229091828.1945072-12-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>