batman-adv: drop batman-adv specific version

Bumping the version number on the first pull request after each merge
window was deemed inappropriate for an in-tree component. The version
number carrie

batman-adv: drop batman-adv specific version

Bumping the version number on the first pull request after each merge
window was deemed inappropriate for an in-tree component. The version
number carries little meaningful information in the context of the Linux
kernel release model, where stable and distribution might all carry
slightly different patches (without any change to the batman-adv version).

Instead, expose a UTS_RELEASE-based string to consumers of the netlink and
ethtool interfaces. To avoid recompilation for each (re)generate of
generated/utsrelease.h, init_utsname()->release is used in code which can
dynamically retrieve the version string. The MODULE_VERSION is moved to a
separate file because it doesn't support dynamic retrieval of the version
string (but constant "at compile time" string) and it is required for the
/sys/module/batman_adv/version. The latter is unfortunately still required
by userspace tools.

Link: https://lore.kernel.org/r/20210203163302.13e8a2a7@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com
Link: https://lore.kernel.org/r/YnfjtpuAaH+Zkf9S@unreal
Link: https://lore.kernel.org/r/Y9faTA0rNSXg%2FsLD@nanopsycho
Signed-off-by: Sven Eckelmann <sven@narfation.org>

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