batman-adv: improve version string

This changes the package version string so it does not start
with "openwrt", but with the base version we are modifying:

So far: openwrt-2019.4-1
Now:    2019.4-openwrt-1

Since it's us modifying version 2019.4 (in this case), this order
is more convenient (and also closer to what the kernel version
string does).

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

batman-adv/Makefile

@@ -76,7 +76,7 @@ NOSTDINC_FLAGS = \
 	-I$(PKG_BUILD_DIR)/include/ \
 	-include backport/backport.h \
 	-include $(PKG_BUILD_DIR)/compat-hacks.h \
-	-DBATADV_SOURCE_VERSION=\\\"openwrt-$(PKG_VERSION)-$(PKG_RELEASE)\\\"
+	-DBATADV_SOURCE_VERSION=\\\"$(PKG_VERSION)-openwrt-$(PKG_RELEASE)\\\"
 
 COMPAT_SOURCES = \
 	$(if $(CONFIG_BATMAN_ADV_MCAST),../../compat-sources/net/core/skbuff.o,) \