scripts: download: coreboot: fix ./download all

When running ./download all, we have the following error:
    resources/scripts/download/coreboot: Line 52: $1 is not set.

The ./download all command was broken by the following commit:
2bb805e2

 (download: Add --help in the
individual download scripts).

Reported-by: madbehaviorus[m] on #libreboot on liberachat
Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>