Distinguish between fs types when mounting sdcards

Currently only vfat formatted sdcards will get mounted, when using other formats like ext4 or btrfs the mount will fail since  unsupported mount options are being passed (uid,gid).
Update mount-sd.sh

Using user id instead of user name and making group explicit.
Check if ID_FS_TYPE is set

I did some extra testing to make sure it all works well and noticed that this script gets called twice (haven't traced it), the first time getting passed an empty ID_FS_TYPE. This change checks if this value is set.
Merge pull request #8 from misja/patch-1

[sd-utils] Distinguish between fs types when mounting sdcards