toolchain-external: Install blackfin FDPIC and FLAT libraries

toolchain-external: Install blackfin FDPIC and FLAT libraries

For certain situations, users may want to install shared FLAT
libraries to the target filesystem even if FDPIC is used as the
primary binary format, or symmetrically users may want to install FDPIC
libraries to the target filesystem even if shared FLAT is used as the
primary binary format.

This commit allows that by:

 * Offering additional Kconfig options to install shared FLAT or FDPIC
   libraries even when those libraries are not selected as the primary
   binary format.

 * Preserving all Blackfin toolchain folders under the
   TOOLCHAIN_EXTERNAL_DIR, instead of keeping only the one related to
   the selected binary format.

 * Adding some additional install targets that do the installation of
   either the shared FLAT or FDPIC libraries when requested.

[Thomas: refactored code, adjusted commit log]
Signed-off-by: Sonic Zhang <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>
Signed-off-by: Peter Korsgaard <[email protected]>