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>

package/base-files/files/usr/lib/os-release

@@ -0,0 +1,17 @@
+NAME="%D"
+VERSION="%V, %N"
+ID="%d"
+ID_LIKE="lede openwrt"
+PRETTY_NAME="%D %N %V"
+VERSION_ID="%v"
+HOME_URL="%m"
+BUG_URL="%b"
+SUPPORT_URL="%s"
+BUILD_ID="%R"
+LEDE_BOARD="%S"
+LEDE_TAINTS="%t"
+LEDE_DEVICE_MANUFACTURER="%M"
+LEDE_DEVICE_MANUFACTURER_URL="%m"
+LEDE_DEVICE_PRODUCT="%P"
+LEDE_DEVICE_REVISION="%h"
+LEDE_RELEASE="%C"