toolchain/external: use generic extract commands (blackfin case)

The backfin toolchains come in two archives.

We extract the first (main) archive using the generic extract commands,
while the second is extracted as a post-extract hook.

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: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>