Longest erase all FLASH for nRF5 series is 295.3 ms for nRF52832.
Timeout period now is set to 340 ms (295.3 + 15%)
Change-Id: Iae00ed7b634f111b9798db11e35e4e066d4aaa95
Signed-off-by: Jānis Skujenieks <janis.skujenieks@gmail.com>
Reviewed-on: http://openocd.zylin.com/4822
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>