batctl: improve version string

batctl: 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>