target: improve robustness of reset command

Before this change jim_target_reset() checked examined state of a target
and failed without calling .assert_reset in particular target layer
(and without comprehensible warning to user).
Cortex-M target (which refuses access to DP under active SRST):
If connection is lost then reset process fails before asserting SRST
and connection with MCU is not restored.
This resulted in:
1) A lot of Cortex-M MCUs required use of reset button or cycling power
after firmware blocked SWD access somehow (sleep, misconfigured clock etc).
If firmware blocks SWD access early during initialization, a MCU could
become completely inaccessible by SWD.
2) If OpenOCD is (re)started and a MCU is in a broken state unresponsive
to SWD, reset command does not work even if it could help to restore communication.
Hopefully this scenario is not possible under full JTAG.

jim_target_reset() in target.c now does not check examined state
and delegates this task to a particular target. All targets have been checked
and xx_assert_reset() (or xx_deassert_reset()) procedures were changed
to check examined state if needed. Targets except arm11, cortex_a and cortex_m
just fail if target is not examined although it may be possible to use
at least hw reset. Left as TODO for developers familiar with these targets.

cortex_m_assert_reset(): memory access errors are stored
instead of immediate returning them to a higher level.
Errors from less important reads/writes are ignored.
Requested reset always leads to a configured action.

arm11_assert_reset() just asserts hw reset in case of not examined target.
cortex_a_assert_reset() works as usual in case of not examined target.

Change-Id: I84fa869f4f58e2fa83b6ea75de84440d9dc3d929
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/2606
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

src/target/mips_m4k.c

@@ -302,6 +302,13 @@ static int mips_m4k_assert_reset(struct target *target)
 	struct mips_m4k_common *mips_m4k = target_to_m4k(target);
 	struct mips_ejtag *ejtag_info = &mips_m4k->mips32.ejtag_info;
 
+	/* TODO: apply hw reset signal in not examined state */
+	if (!(target_was_examined(target))) {
+		LOG_WARNING("Reset is not asserted because the target is not examined.");
+		LOG_WARNING("Use a reset button or power cycle the target.");
+		return ERROR_TARGET_NOT_EXAMINED;
+	}
+
 	LOG_DEBUG("target->state: %s",
 		target_state_name(target));