net: tftp: Add IPv6 support for tftpboot

The command tftpboot uses IPv4 by default. Add the possibility to use IPv6
instead. If an address in the command is an IPv6 address it will use IPv6
to boot or if there is a suffix -ipv6 in the end of the command it also
force using IPv6. All other tftpboot features and parameters are left
the same.

Signed-off-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>