improve startup tcl scripts

Fix a couple of layering violations missed in the last round.
Add missing comment headers.

src/jtag/startup.tcl

@@ -9,6 +9,21 @@ proc jtag_init {} {
 	}
 }
 
+# This reset logic may be overridden by board/target/... scripts as needed
+# to provide a reset that, if possible, is close to a power-up reset.
+#
+# Exit requirements include:  (a) JTAG must be working, (b) the scan
+# chain was validated with "jtag arp_init" (or equivalent), (c) nothing
+# stays in reset.  No TAP-specific scans were performed.  It's OK if
+# some targets haven't been reset yet; they may need TAP-specific scans.
+#
+# The "mode" values include:  halt, init, run (from "reset" command);
+# startup (at OpenOCD server startup, when JTAG may not yet work); and
+# potentially more (for reset types like cold, warm, etc)
+proc init_reset { mode } {
+	jtag arp_init-reset
+}
+
 #########
 
 # TODO: power_restore and power_dropout are currently neither