tcl: add a tclsh symlink

tclsh is installed with its version number (tclshx.y) but scripts (like
usb_modeswitch_dispatcher) are calling tclsh, not tclshx.y.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>