factory_ExternalStorage: fix ARM device names

ARM platforms have MMC device name in "mmcblk*" form (although USB stick is
still in sdX form).  We need to support that kind of devices.

TEST=in factory test, run USB and SD test; both sucess.

Change-Id: I13a624f3d00159f91f9b835316dada647f938a97
Cherry-Pick: http://gerrit.chromium.org/gerrit/750
Reviewed-on: http://gerrit.chromium.org/gerrit/753
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
1 file changed