Add 'netserver' command to EFI loader.

In some environments is difficult to access bootp/dhcp
configuration as "standard user". Add a command that allows to set
or display the URI of the network server used as "net:" 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

Reviewed by: imp, kevans
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D26736