base-files: Add standard os-release file

/etc/os-release is the standard distribution release information
file, therefore add it (and image configuration options for
fields not previously present in LEDE).  Once it is deemed
reasonable the non-standard openwrt_release, openwrt_version,
and device_info files could be removed (that is with this patch
we consider them deprecated in favour of the standard file).

Signed-off-by: Daniel Dickinson <lede@daniel.thecshore.com>