build: Use git submodules for Fuel git repos

This change reworks support for cloning (fetching) and patching all
Fuel components (fuel-library ... fuel-ui, see [1] for full list),
both outside the ISO build process (for development purposes, like
testing OPNFV patches apply cleanly to each component), and during
the ISO build.

The implementation relies heavily on git submodules and patches,
so backporting pending upstream gerrit changes, as well as adding new
out-of-upstream-tree patches, should be trivial.

Also, since without tracking remotes the repos rarely change, leverage
the in-place OPNFV build caching mechanism to drastically reduce the
number of git clones during each build.

The mechanism is detailed more in-depth in the JIRA ticket [3], and
it's merely a rehash of the submodule handling in Armband, implemented
initially by Stanislaw Kardach <stanislaw.kardach@cavium.com>.

Pros (+ new features):
 - OPNFV patches for Fuel components can be applied (imported) or
   developed (exported) outside of the ISO build process;
 - git repo caching reduces the number of external pulls;
 - reuse the same fuel-* componets in ALL targets
   e.g. fuel-mirror used to employ 2 slightly different git trees;
 - one step closer to Fuel/Armband merge (lots of steps left though);

Cons:
 - adds a lot complexity to solve a non-problem (handling the repos
   is fine in the current form, for the current goals);
   However, patching <fuel-astute> or <fuel-agent> seems to be
   quite complicated with the current mechanism, as env vars
   override our locally patched repos for these components).
   The proposed change should eliminate this issue completely.

CHANGE:
 Aligned divergent fuel-mirror in ISO vs local repo build by
 applying the 302 redirect fix ("Fixed handling http redirects") [2]
 on top of 9.0.1 tag in fuel-mirror repo, and using that for both.
 This obsoleted the followking mk file:
 - build/f_isoroot/f_repobuild/config.mk

CHANGE:
 Removed unused make target `get-fuel-repo` from build/config.mk.

FIXME:
 Remote tracking is currently deactivated for all branches.
 Change this when remote HEAD should be tracked instead,
 e.g. during Newton rebase.

[1] https://github.com/openstack/fuel-main/blob/stable/mitaka/repos.mk#L32
[2] https://review.openstack.org/#/c/334882/
[3] https://jira.opnfv.org/browse/FUEL-200

TODO: Extend build/f_repos/README.md with info about:
 - branch changes;
 - tag bumps;
 - patching now supports subdirs;

v5 -> v6:
 * Added support for nesting patches in subdirs, to be used for feature
   groups and related changes across submodules.
   E.g. Upcoming multiarch EFI support could go in:
    - .../f_repos/patch/fuel-agent/multiarch-efi/...;
    - .../f_repos/patch/fuel-astute/multiarch-efi/...;
   While Armband patches could go in:
    - .../f_repos/patch/fuel-agent/armband/...;
    - .../f_repos/patch/fuel-astute/armband/...;
   etc.
v6 -> v13:
 * fix wrong tag used for VERSION dump;
 * fix wrong tag used for patches-export;
 * move OPNFV_TAG to main config.mk and use it in FUEL_*_COMMIT;
 * only run `patches-import` when patches changed / not yet applied;
 * only run `clean` for stale `.cacheid` (avoid re-applying patches);
 * allow git submodule path/name to be different (FIXED);
 * put-cache should only run when artifact is not already cached;
 * `git am`: use 3-way merge so already applied patches are gently
   skipped (useful for upstreaming patches from Armband);
 * Properly support nested patch dirs;
 * Colorize the output a little;

JIRA: FUEL-200

Change-Id: I9dbdd9b3022896d4497d21be8dacc859730db489
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

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