linux-bbone: update to 4.1.12-ti-r28
linux-bbone: update to 4.1.12-ti-r28

use a mirror for sources and drop a note why we do so

Signed-off-by: Andreas Müller <d72a15aaaca3a7afa855175c72dc8d2e05931e32@googlemail.com>

schnitzeltony/meta-bbone