toolchain-external: rename Blackfin related special variables

toolchain-external: rename Blackfin related special variables

As suggested by Arnout, this commit renames:

 - TOOLCHAIN_EXTERNAL_INSTALL_BFIN_FDPIC to
   TOOLCHAIN_EXTERNAL_INSTALL_TARGET_BFIN_FDPIC

 - TOOLCHAIN_EXTERNAL_INSTALL_BFIN_FLAT to
   TOOLCHAIN_EXTERNAL_INSTALL_TARGET_BFIN_FLAT

Which makes it clear that those variables are installing libraries to
the target, and make their naming more consistent with the naming of
other variables in the file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>