oeqa/sdk/sdkbuildproject: use os.path.join

Instead of mushing two paths together and hoping that the slashes line up
correctly, use os.path.join.

(From OE-Core rev: ed0413babaf10ccf729472f603bbe32d84ba1931)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/sdk/utils/sdkbuildproject.py

@@ -20,10 +20,9 @@ class SDKBuildProject(BuildProject):
         BuildProject.__init__(self, uri, foldername, tmpdir=testpath, dl_dir=dl_dir)
 
     def download_archive(self):
-
         self._download_archive()
 
-        cmd = 'tar xf %s%s -C %s' % (self.targetdir, self.archive, self.targetdir)
+        cmd = 'tar xf %s -C %s' % (os.path.join(self.targetdir, self.archive), self.targetdir)
         subprocess.check_output(cmd, shell=True)
 
         #Change targetdir to project folder