Reset u-boot download timeout with every '#'

Reset u-boot download timeout with every '#'

Previously, we used two consecutive hash characters, but it turned out
that a Raspberry Pi, on our network experiences few timeouts at the
beginning (see '#T ' sequences below) and there are never two
consecutive hashes before the timeout elapses.

    U-Boot> tftpboot ${kernel_addr_r} /rpi/kernel
    Using ethernet@7d580000 device
    TFTP from server 10.35.95.2; our IP address is 10.35.95.45
    Filename '/rpi/kernel'.
    Load address: 0x80000
    Loading: #T #T T T Kernel load: timeout at /home/wsh/.nix-profile/bin/novaboot line 1298, <> line 7.

Therefore, we reset the timeout after just a single hash. Now the
above example runs as shown below:

    U-Boot> tftpboot ${kernel_addr_r} /rpi/kernel
    Using ethernet@7d580000 device
    TFTP from server 10.35.95.2; our IP address is 10.35.95.45
    Filename '/rpi/kernel'.
    Load address: 0x80000
    Loading: #T #T T ################################################  45 MiB
             2 MiB/s
    done