efi: change 'env -e -i' usage syntax

'env -e -i' syntax was changed from "," to ":". Account for this also
in the documentation.

Fixes: 2b3fbcb5 ("efi_loader: use ':' as separator for setenv -i")
Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Correct the usage description for setenv -e too.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>