target/cortex_m: supress historical reset detection

target/cortex_m: supress historical reset detection

The S_RESET_ST sticky bit is reset after DHCSR read.
It is set at power-on reset and keeps active until the debuger reads DHCSR.

Ignore S_RESET_ST at the very first read after OpenOCD start
and suppress possibly misleading message "external reset detected"
if we cannot guarantee the reset happened recently.

While on it add a TODO comment.

Change-Id: I15217c2ca6f69ac97aff8be86bce67cba94a42cd
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7109
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>