The toolchain-scripts class was changed to assign to the variable using ??=
which allows users to override it. We therefore need to _append instead
of += to have the change take effect as intended.
(From OE-Core rev: d589777a4d423b1f0c9953900bd79772d15ee4af)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>