hikirk: set discard option for emmc mount

Set mount option discard for emmc. Operation system informs emmc which blocks
of data are no longer in use. This should increase the emmc life.

Signed-off-by: Eik Binschek <eik.binschek@dresearch-fe.de>
Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>