If the user has set a u-boot env var named rootpath, automatically
import it into the loader(8) env as dhcp.root-path, so that it overrides
any dhcp/bootp server-provided path.

Now if you have a dhcp server available you can easily net-boot a u-boot
system even if you don't control the dhcp server config, by setting just
two variables in the u-boot env:

loaderdev=net
rootpath=<nfsserverip>:<pathname>

Previously you had to either accept all the dhcp parameters from the
server without the ability to locally provide the rootpath, or you had
to forego dhcp and set more vars (ipaddr, netmask, serverip, rootpath).