Allow to change the path between OSMOCOM_REPO_MIRROR and
OSMOCOM_REPO_VERSION. While at it, tweak related comments (comment above
the variable as usually, replace "repo" wording with "feed" for the
latest/nightly variable as we usually refer to it as feed), and mention
OSMOCOM_REPO_* in README.md.
In order to be able to use a different mirror for testsuite and systems
under test, the testsuite related Dockerfiles (osmocom-bb-host-master,
debian-stretch-titan) are not using OSMOCOM_REPO_PATH. We could add a
OSMOCOM_REPO_TESTSUITE_PATH on demand, as mentioned in the next commit.