Due to iteration over possibly multiple variables, the return value of
an individual call to nvvar_remove() / nvvar_add() gets lost.

This will result in do_nv() returning success (0) if any variable
processing failed as long as processing of the last variable succeeded.

Processing will not be aborted on individual errors, such as the 'cp'
handles it for invalid files.

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