mbedtls: use correct SPDX License Identifier and add License file

License "GPL-2.0+" is deprecated License Identifier according to
SPDX License list [1]. The correct one is GPL-2.0-or-later.
While at it, also add the License file.

[1] https://spdx.org/licenses/

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

package/libs/mbedtls/Makefile

@@ -17,7 +17,8 @@ PKG_SOURCE_URL:=https://tls.mbed.org/download/
 PKG_HASH:=6ebdea6565c714f1315b9af6a802afb4b4e89976f7d5d2b15aa8028eb52e7d09
 
 PKG_BUILD_PARALLEL:=1
-PKG_LICENSE:=GPL-2.0+
+PKG_LICENSE:=GPL-2.0-or-later
+PKG_LICENSE_FILES:=gpl-2.0.txt
 PKG_CPE_ID:=cpe:/a:arm:mbed_tls
 
 PKG_CONFIG_DEPENDS:=CONFIG_LIBMBEDTLS_DEBUG_C