oonf-dlep-radio: refactor Makefile (package versioning, downloading)

oonf-dlep-radio: refactor Makefile (package versioning, downloading)

- By this change, we are using downloading .tar.xz (which is default in OpenWrt)
Reference: OpenWrt base repository include/download.mk

Downloaded tarball size is ~60 kB smaller.

- Removed "v" in package naming.
Before: oonf-dlep-radio_v0.15.1_arm_cortex-a9_vfpv3-d16.ipk
After: oonf-dlep-radio_0.15.1_arm_cortex-a9_vfpv3-d16.ipk

Removed stuff, which is no longer needed like PKG_REV.

- Switch to AUTORELEASE in PKG_RELEASE

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