fix(arm): don't race on the build directory

Wait for it to have been created. This is the same issue as
commit db69d118294f08aae86378c98aa082ac73e15b73.

Change-Id: I32bd0c713e2837563d32131fb0beddb5533c0792
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
1 file changed