add overridable Tcl "init_reset"

This abstracts the "jtag arp_init-reset" call into a method
called from OpenOCD startup and reset processing.

Platforms which have different requirements for how such hard
resets must be performed can now override "init_reset" instead
of needing to rebuild custom hacked versions of the server.

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