Fix bios-flash-offset for tinyFPGA

[hubmartin]

I was unable to run BIOS until I fixed bios-flash-offset to 0x50000.

Not sure where the original value 0x60000 comes from, but tinyprog on my kit shows that userdata area is at 0x50000.
Could it be that there are different BX variants?

$litex tinyprog -m
[
  {
    "boardmeta": {
      "name": "TinyFPGA BX",
      "fpga": "ice40lp8k-cm81",
      "hver": "1.0.0",
      "uuid": "8503d354-fcf6-4c92-9e16-016e3e7f229a"
    },
    "bootmeta": {
      "bootloader": "TinyFPGA USB Bootloader",
      "bver": "1.0.1",
      "update": "https://tinyfpga.com/update/tinyfpga-bx",
      "addrmap": {
        "bootloader": "0x000a0-0x28000",
        "userimage": "0x28000-0x50000",
        "userdata": "0x50000-0x100000"
      }
    },
    "port": "/dev/ttyACM0"
  }
]

Also tried to pass argument --bios-flash-offset=0x50000 but even when I set it hexadecimal or decimal number, I always get python error that this parameter is a string and cannot be used with +. However this is just a question out of scope of this PR.

[enjoy-digital]

Thanks @hubmartin, I'm also not sure why 0x60000 was used. Your PR is merged and the issue with --bios-flash-offset is fixed with 8664b59.