rename jtag_khz as adapter_khz

Globally rename "jtag_khz" as "adapter_khz", and move it out of the "jtag"
command group ...  it needs to be used with non-JTAG transports

Includes a migration aid (in jtag/startup.tcl) so that old user scripts
won't break.  That aid should Sunset in about a year.  (We may want to
update it to include a nag message too.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

tcl/interface/usb-jtag.cfg

@@ -7,5 +7,5 @@
 interface usb_blaster
 usb_blaster_vid_pid 0x16C0 0x06AD
 usb_blaster_device_desc "USB-JTAG-IF"
-jtag_khz 3000
+adapter_khz 3000