flex: delete broken flex++ symlink from target

When flex is built for the target without installing the
flex binary, a flex++ symlink installed by flex's Makefile
points to the missing flex executable.  This mod adds
a post target install hook to remove the broken symlink.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>