toolchain-external: fix support for non-ADI Blackfin external toolchains

The computation of TOOLCHAIN_EXTERNAL_BIN has a special case for
Blackfin, where it's set to
$(TOOLCHAIN_EXTERNAL_INSTALL_DIR)/$(TOOLCHAIN_EXTERNAL_PREFIX)/bin
instead of $(TOOLCHAIN_EXTERNAL_INSTALL_DIR)/bin for other
architectures.

However, this is actually only true for Analog Devices pre-built
toolchains. Other Blackfin external toolchains (such as ones built by
Buildroot) do not have this special organization.

Therefore, in order to make those non-ADI Blackfin toolchains work, we
need to change the condition from BR2_bfin to testing specifically for
the ADI toolchains.

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