toolchain-external/blackfin: drop --hard-dereference

Currently, for the blackfin external toolchains, we tell tar to
extract files with the --hard-dereference. However, --hard-dereference
is only meaningful when creating an archive, not when extracting
it. Therefore, let's drop this option.

[Thomas: rework commit title and commit log, after some suggestions
from Arnout.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>