🐛 book: fix make serve on MacOS

[sbueringer]

Signed-off-by: Stefan Büringer [email protected]

What this PR does / why we need it:
realpath doesn't exist on MacOS (at least on a clean system). So I got the following errors before this fix:

$ mdbook serve
2021-10-06 17:33:41 [INFO] (mdbook::book): Book building has started
./util-embed.sh: line 21: realpath: command not found
./util-releaselink.sh: line 21: realpath: command not found
./util-tabulate.sh: line 21: realpath: command not found

This leads to rendering issues in e.g. the quickstart page.

This PR replaces realpath with something which should work on MacOS and on Linux (we're also already calculating the repo root the same way in our ci scripts).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[vincepri]

This works for me on Mac if you have coreutils installed

[vincepri]

/approve
/lgtm

[k8s-ci-robot]

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