Because unfs boot mode is unstable for qemuppc, disable
it temporarily.
(From OE-Core rev: 74ff1bc8c248824116ba4b787b10fa6ee0c13ce1)
Signed-off-by: Liming Wang <liming.wang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>