update sd image tool

umount the 3rd partiton of the SD card,if the partiton was auto mounted
by system,genrerate sd image will fail

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>
sd image: fix the bug that system cannot syspend to mem

mediaserver should have the system privilege to access the autosleep
sys node

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>