base-files: use OPENWRT prefix for os-release variables

Just stumbled across this LEDE legacy, without finding any real reason
to keep it.  There is a single LEDE_DEVICE_MANUFACTURER_URL dependency
in the luci feed repo which needs to be syncronized.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
[re-added missing commit message]
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 8a34a54b6aa6e9a5e2983d554fd5b97bec97e891)

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

@@ -8,11 +8,11 @@ HOME_URL="%u"
 BUG_URL="%b"
 SUPPORT_URL="%s"
 BUILD_ID="%R"
-LEDE_BOARD="%S"
-LEDE_ARCH="%A"
-LEDE_TAINTS="%t"
-LEDE_DEVICE_MANUFACTURER="%M"
-LEDE_DEVICE_MANUFACTURER_URL="%m"
-LEDE_DEVICE_PRODUCT="%P"
-LEDE_DEVICE_REVISION="%h"
-LEDE_RELEASE="%D %V %C"
+OPENWRT_BOARD="%S"
+OPENWRT_ARCH="%A"
+OPENWRT_TAINTS="%t"
+OPENWRT_DEVICE_MANUFACTURER="%M"
+OPENWRT_DEVICE_MANUFACTURER_URL="%m"
+OPENWRT_DEVICE_PRODUCT="%P"
+OPENWRT_DEVICE_REVISION="%h"
+OPENWRT_RELEASE="%D %V %C"