cmd_nvedit: Make 'env import -c' require size parameter

When importing a checksummed area we need to be told how big the area in
question is so that we know that will match the size of the area which
the checksum is generated against.

Reported-by: Pierre AUBERT <p.aubert@staubli.com>
Signed-off-by: Tom Rini <trini@ti.com>