tftp: make TFTP ports unconditionally configurable

tftp: make TFTP ports unconditionally configurable

A few lines of code being guarded by the CONFIG_TFTP_PORT option seems
an unnecessary restriction on the TFTP support provided by a vanilla
U-Boot image. In cases where the TFTP server cannot run as superuser -
and hence cannot run on the well-known port 69 - this quirk incurs a
full reconfiguration and rebuild of the bootloader only in order to
select the appropriate destination port.

Remove the CONFIG_TFTP_PORT option entirely and make the tftpdstp and
tftpsrcp variables always have an effect. Their being unset will mean
that U-Boot behaves the same as if CONFIG_TFTP_PORT was unset. Update
the documentation accordingly. And fix up the single board which was
originally enabling this option.

Signed-off-by: Alvin Šipraga <alvin@pqrs.dk>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>