sysfsgpio: give time to udev to change gpio permission

sysfsgpio: give time to udev to change gpio permission

When a gpio is exported by writing in /sys/class/gpio/export, the
corresponding gpio control files appear immediately in sysfs but
with default access permission for root user only. The daemon udev
requires some time to get notified of the new files before it can
change the permissions to allow access to unprivileged users.
Due to this race condition, sysfsgpio can fail with EACCES error
if OpenOCD is executed by any unprivileged user.

Give 0.5 seconds to udev to identify the new files and change the
permission.

Tested with udev rules:
	SUBSYSTEM=="gpio*", PROGRAM="/bin/sh -c 'find -L /sys/class/gpio/ -maxdepth 2 -exec chown root:uucp {} \; -exec chmod g=u {} \; || true'"

Change-Id: I1316c66ff103ffe23e5e4720f33372dc272a3766
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5302
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>