more tcl/{board,target} cleanup

more tcl/{board,target} cleanup

Remove more remnants of the old "jtag_device" syntax.

Don't [format "%s.cpu" $_CHIPNAME] ... it's needless complexity.

Remove various non-supported "-variant" target options; they're not
needed often at all.

Flag some of the board files as needing to have and use target files
for the TAP and target declarations.

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