image: Avoid race over directory creation

There is a race over the do_package_qa task and the do_rootfs task since rootfs recreates a directory. This patch disables the task (which isn't used for images) to avoid the race:

NOTE: recipe core-image-minimal-1.0-r0: task do_package_qa: Started
NOTE: recipe core-image-minimal-1.0-r0: task do_rootfs: Started
ERROR: Build of do_package_qa failed
ERROR: Traceback (most recent call last):
File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-mips/build/bitbake/lib/bb/build.py", line 497, in exec_task
return _exec_task(fn, task, d, quieterr)
File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-mips/build/bitbake/lib/bb/build.py", line 440, in _exec_task
exec_func(func, localdata)
File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-mips/build/bitbake/lib/bb/build.py", line 212, in exec_func
exec_func_python(func, d, runfile, cwd=adir)
File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-mips/build/bitbake/lib/bb/build.py", line 237, in exec_func_python
os.chdir(cwd)
OSError: [Errno 2] No such file or directory: '/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-mips/build/build/tmp/work/qemumips-poky-linux/core-image-minimal/1.0-r0/core-image-minimal-1.0'

index 82e977c..832c738 100644
@@ -409,6 +409,7 @@ do_compile[noexec] = "1"
 do_install[noexec] = "1"
 do_populate_sysroot[noexec] = "1"
 do_package[noexec] = "1"
+do_package_qa[noexec] = "1"
 do_packagedata[noexec] = "1"
 do_package_write_ipk[noexec] = "1"
 do_package_write_deb[noexec] = "1"