Document the configuration format

[yujqiao]

In the setup.md, boot.cfg is mentioned multiple times. But I have no idea what that is and it seems to be undocumented.

[et-tommythorn]

Thanks, that very true, the documentation could be better.

FWIW, the documentation does say cd <dromajo>/run and inside run you find boot.cfg, which is currently

{
    "version":1,
    "machine":"riscv64",
    "memory_size":1024,
    "bios":"fw_jump.bin",
    "kernel":"Image",
    "initrd":"rootfs.cpio",
    "cmdline": "root=/dev/ram rw earlycon=sbi console=hvc0 bench=spec06_gcc",
    "memory_base_addr":0x80000000
}

The configuration format was inherited from RISCVEMU, but we are slowly moving away from it and you can do most things without it.