Add context for persist filesystem.

Label perist firmware with seandroid context

Signed-off-by: 秋叶随风ivan <yanziily@gmail.com>
sepolicy: Update

type=1400 audit(1408923284.900:4): avc:  denied  { read } for  pid=205 comm="ueventd" name="adsp.mdt" dev="mmcblk0p23" ino=3 scontext=u:r:ueventd:s0 tcontext=u:object_r:sdcard_external:s0 tclass=file
type=1400 audit(1408923284.900:5): avc:  denied  { open } for  pid=205 comm="ueventd" name="adsp.mdt" dev="mmcblk0p23" ino=3 scontext=u:r:ueventd:s0 tcontext=u:object_r:sdcard_external:s0 tclass=file
type=1400 audit(1408923284.900:6): avc:  denied  { getattr } for  pid=205 comm="ueventd" path="/firmware/image/adsp.mdt" dev="mmcblk0p23" ino=3 scontext=u:r:ueventd:s0 tcontext=u:object_r:sdcard_external:s0 tclass=file

Signed-off-by: 秋叶随风ivan <yanziily@gmail.com>