oeqa/selftest/bblock/fitimage/sstatetests: Fix changing MACHINE during the test

oeqa/selftest/bblock/fitimage/sstatetests: Fix changing MACHINE during the test

With config fragments, changing MACHINE in the test like this no
longer works. Use the forcevarable override to allow it to work.
This also needs a tweak to bitbake to work correctly, sent
seperately. Whilst ugly, this avoids the need to start changing
config fragments within oeqa right now.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>