Changes

asterisk-master: Use pjproject from external git repo

By default, asterisk.git build system will fetch pjproject tarball from
https://raw.githubusercontent.com/asterisk/third-party/master/pjproject/$(PJPROJECT_VERSION),
where PJPROJECT_VERSION is defined in asterisk/third-party/versions.mak:
PJPROJECT_VERSION=2.14
On top of that, it applies some patches adapting build system, found in
asterisk/third-party/pjproject/patches/.

The build system also allows to use an out-of-tree pjproject (eg from a
git repo) by symlinking the directory to
asterisk/third-party/pjprojec/source/.

This is all documented in asterisk/third-party/pjproject/README-hacking.md.

Since we want to develop on pjproject too, fetch our own pjproject.git
repo, where we can push changes, and build asterisk using that repo.

Bear in mind that, when asterisk.git builds with out-of-tree pjproject,
it wont apply pjproject patches under asterisk.git/third-party/pjproject/patches.
Hence, I created an "asterisk/2.14" branch in our pjproject.git repo which points
to tag "2.14" (the one used by asterisk 20.7.0, which we use), and has the 2 patches
applied on top.
Then, we have our own "sysmocom/2.14" branch on top of that "asterisk/2.14"
branch, where we put our development. WIP development can also be done  in whatever
branch  on top of "asterisk/2.14" branch.

Change-Id: I73f3323db1b6897fc43215578b0a011fdb6909c5