stlink: add armv7m stlink handling

This enables us to better handle some of the low level functions that the
stlink does not support. It also enables us to share a few more of the
standard cortex_m3 functions if necessary.

Change-Id: I7a2c57450122012ec189245d8879d8967913e00e
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/637
Tested-by: jenkins

src/target/arm_adi_v5.c

@@ -1096,14 +1096,6 @@ int ahbap_debugport_init(struct adiv5_dap *dap)
 
 	LOG_DEBUG(" ");
 
-	/* test for initialized low level jtag hardware
-	 * this always fails for stlink hardware
-	 */
-	if (!dap->jtag_info) {
-		LOG_DEBUG("No low level jtag hardware found");
-		return ERROR_OK;
-	}
-
 	/* JTAG-DP or SWJ-DP, in JTAG mode
 	 * ... for SWD mode this is patched as part
 	 * of link switchover