Add 'netserver' command to EFI loader.

[mmel]

Summary

In some environments is difficult to access/reconfigure bootp/dhcp
configuration as "standard user". Add a command that allows to set the URI
of the network server used as a reference to the "network" device. Currently
only tftp and nfs protocols are supported.

Typical usage pattern is:

netserver tftp://192.168.168.1/path_to_obj_dir/arm.armv7/sys/GENERIC/
boot net:kernel