build_image: use braces on vars & [[...]] for tests

A minor clean up over CL:266722 to match our style guide.

BUG=brillo:856
TEST=`bash -n build_image` passes

Change-Id: I368237887017db49a56f5acf3334f520ab737bfa
Reviewed-on: https://chromium-review.googlesource.com/267420
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Steve Fung <stevefung@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed