We should be able to test sdks in parallel. As far as I can tell, this
restriction crept in for no good reason, based on locks that testimage used
for the non-qemu case.
Therefore remove the lock and allow sdks to test in parallel.
(From OE-Core rev: 54477664dc13c6aa15b3246f328b66591a340659)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>