discover/grub2: Fix behavior of save_env -f

discover/grub2: Fix behavior of save_env -f

Currently, "save_env -f" in the GRUB2 parser only works with three
arguments, which means only commands of the form "save_env -f <path>"
that save *no* environment variables are allowed.

Allow "save_env -f <path> [<var>]*", making "save_env -f" useful.

Tested:
 Unit test test-grub2-save-env-dash-f tests this change, and the
 remaining unit tests still pass.

Signed-off-by: Alan Dunn <amdunn@google.com>
Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>