devtool: add support for git submodules

devtool: add support for git submodules

Adding the support of submodules required a lot of changes on the
internal data structures:
* initial_rev/startcommit used as a starting point for looking at new
  / updated commits was replaced by a dictionary where the keys are the
  submodule name ("." for main repo) and the values are the
  initial_rev/startcommit

* the extractPatches function now extracts patch for the main repo and
  for all submodules and stores them in a hierarchical way describing the
    submodule path

* store initial_rev/commit also for all submodules inside the recipe
  bbappend file

* _export_patches now returns dictionaries that contains the 'patchdir'
  parameter (if any). This parameter is used to add the correct
  'patchdir=' parameter on the recipe

Also, recipe can extract a secondary git tree inside the workdir.

By default, at the end of the do_patch function, there is a hook in
devtool that commits everything that was modified to have a clean
repository. It uses the command: "git add .; git commit ..."

The issue here is that, it adds the secondary git tree as a submodule
but in a wrong way. Doing "git add <git dir>" declares a submodule but do
not adds a url associated to it, and all following "git submodule foreach"
commands will fail.

So detect that a git tree was extracted inside S and correctly add it
using "git submodule add <url> <path>", so that it will be considered as a
regular git submodule

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>