jtag/tcl.c cleanup -- split out "adapter.c"

Clean up the jtag/tcl.c file, which was one of the biggest and
messiest ones in that directory.  Do it by splitting out all the
generic adapter commands to a separate "adapter.c" file (leaving
the "tcl.c" file holding only JTAG utilities).

Also rename the little-used "jtag interface" to "adapter_name", which
should have been at least re-categorized earlier (it's not jtag-only).

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

src/jtag/startup.tcl

@@ -78,7 +78,7 @@ proc srst_asserted {} {
 
 # BEGIN MIGRATION AIDS ...  these adapter operations originally had
 # JTAG-specific names despite the fact that the operations were not
-# specific to JTAG.
+# specific to JTAG, or otherewise had troublesome/misleading names.
 #
 # FIXME phase these aids out after about April 2011
 #