generic: routerboot sysfs: move tag_show_string()

This routine will be shared between hard and soft config drivers.
Also use scnprintf() instead of snprintf().

Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Tested-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>