Create X symlink for TinyX

When installing TinyX to target create /usr/bin/X symlink
to Xfbdev as it is the only TinyX driver we build.
This makes utils like startx happy :)

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>