toolchain-external: blackfin: install FDPIC libraries also to staging

toolchain-external: blackfin: install FDPIC libraries also to staging

For external Blackfin toolchains with BR2_BFIN_INSTALL_FDPIC_SHARED set,
the FDPIC shared libraries are currently only copied to the target
directory, not to staging.

For debugging purposes, an unstripped copy in staging is necessary.
Moreover, this change will simplify a subsequent change that lines up the
location of shared libraries between target and staging directories.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>