exfat: Create symbolic link /sbin/mount.exfat

Currently using `udisks --mount /dev/sdb1` with an exFAT partition,
currently doesn’t work.

Using `strace`, it looks like, `mount` only looks in `/sbin`.

```
$ sudo strace mount /scratch/local/exfat.img /scratch/local/mnt2
[…]
munmap(0x7f53721c7000, 266240)          = 0
stat("/sbin/mount.exfat", 0x7fffaf76fa60) = -1 ENOENT (No such file or directory)
stat("/sbin/fs.d/mount.exfat", 0x7fffaf76fa60) = -1 ENOENT (No such file or directory)
stat("/sbin/fs/mount.exfat", 0x7fffaf76fa60) = -1 ENOENT (No such file or directory)
lstat("/run/mount/utab", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
open("/run/mount/utab", O_RDWR|O_CREAT|O_CLOEXEC, 0644) = 3
close(3)
[…]
```

So create a symbolic link. It works then.

exfat.be0

@@ -46,3 +46,8 @@ mee_patch_post() {
 #mee_install() {
 #    bee_install
 #}
+
+mee_install_post() {
+    start_cmd mkdir ${D}/sbin
+    start_cmd ln -s ${D}/usr/sbin/mount.exfat ${D}/sbin/
+}