complete: Add completion for nv and globalvar commands ...

The 'nv' command is often used to create a nv variable
for an existing global variable, so add a command completion
function for this.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

nv: Allow to set/remove multiple variables with one command ...

It's convenient to set/remove multiple nv variables in one go. With
this patch we iterate over the remaining nonopts instead of expecting
exactly one nonopt.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

nv: Add option to explicitly save nv variables ...

We now have code to save the nv variables without saving the
rest of the environment. This gives us the possibility to explicitly
save the nv variables. This patch adds an option to the 'nv' command
for this.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add support for non volatile variables ...

This adds (back) support for non volatile variables. Non volatile
variables are variables which are stored in the environment over
reboot. They are used in the same way as the global variables, but
with a 'nv' command and device. The variables are stored under
/env/nv/, one variable per file. Adding a nv variable automatically
adds a global variable with the same name. Changing a nv variable
also changes the same global variable, but not the other way round.
This allows for example to configure the username as:

nv user=sha; saveenv

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>