Currently we can choose between 2 different types of behavior for the
serverip variable:

  1) Always overwrite it with the DHCP server IP address (default)
  2) Ignore what the DHCP server says (CONFIG_BOOTP_SERVERIP)

This patch adds a 3rd option:

  3) Use serverip from DHCP if no serverip is given
     (CONFIG_BOOTP_PREFER_SERVERIP)

With this new option, we can have the default case that a boot file gets
loaded from the DHCP provided TFTP server work while allowing users to
specify their own serverip variable to explicitly use a different tftp
server.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>