toolchain: kill ADI Blackfin toolchain

This toolchain has many problems which are fixed in contemporary gcc
and uClibc-ng. In addition, several hacks are needed to be able to
work with this toolchain. All these hacks are removed as well.  Also
the package exceptions for this toolchain are removed.

The BR2_BFIN_INSTALL_FDPIC_SHARED and BR2_BFIN_INSTALL_FLAT_SHARED
options don't get a legacy entry. For the ADI toolchain, there already
is a legacy entry, so it doesn't make sense to add it twice. For other
external toolchains, these options didn't actually work, because they
rely on the specific layout of the ADI toolchain.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>