toolchain: disable internal for blackfin

The internal toolchain was a "best effort" approach - we strived to make
it build properly and all but it's mostly untested.
Since it's got issues disable it until it's properly fixed and tested
and leave the official ADI toolchain instead.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>