[ncarrier]

The tcl interpreter is :
* hardcoded to jimsh when producing usb_modeswitch.string
* autodetected, falling back to a hand compiled jimsh version in find-tclsh
* autodetected, falling back to a hand compiled jimsh version in make-static-dispatcher.sh
* autodetected, when producing the usb_modeswitch.tcl shebang

This patch :
* defines HOST_TCL as the tcl shell used to product make targets
* defines TCL as the tcl shell used by the usb_modeswitch_dispatcher tcl script
* allows user to override both two if needed

I tried to preserve the previous heuristic used for the target tcl interpreter choice, i.e., reasonable candidates are searched on host PATH, falling back to /usr/bin/tclsh if none were to be found (i.e. find-tclsh returned [...]jimsh0).