hw/riscv/boot.c: introduce riscv_default_firmware_name()

hw/riscv/boot.c: introduce riscv_default_firmware_name()

Some boards are duplicating the 'riscv_find_and_load_firmware' call
because the 32 and 64 bits images have different names. Create
a function to handle this detail instead of hardcoding it in the boards.

Ideally we would bake this logic inside riscv_find_and_load_firmware(),
or even create a riscv_load_default_firmware(), but at this moment we
cannot infer whether the machine is running 32 or 64 bits without
accessing RISCVHartArrayState, which in turn can't be accessed via the
common code from boot.c. In the end we would exchange 'firmware_name'
for a flag with riscv_is_32bit(), which isn't much better than what we
already have today.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Message-Id: <20221221182300.307900-6-dbarboza@ventanamicro.com>
Message-Id: <20221229091828.1945072-11-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>