tools: generate_vsn: remove bashism and use POSIX /bin/sh

[kaniini]

This PR addresses #1988

Proposed changes include:

• removal of dependency on /bin/bash for generating the version number, use POSIX /bin/sh

[arcusfelis]

fine.

But both versions assume, that we are running from one of directories of the repo.
Would still work differently for both versions if called from repo and from outside of repo.

[arcusfelis]

would replace with something like:

#!/bin/sh

# Go to repo root
cd `dirname "$0"`/../

GIT_VSN=`git describe --always --tags 2>/dev/null`
if [ $? -eq 0 ]; then
    echo $GIT_VSN
else
    cat VERSION
fi

[michalwski]

@arcusfelis thanks for the tip. I applied it and merge the PR into master.

[michalwski]

@kaniini, thanks for you contribution. I merged your PR!