Changes

make/Makefile: use git to get the project's root directory

Change-Id: I87a425515692e62df7b593a3c2a5b88a765d217d

Make respawn.sh a common part of *-build/-obs images

Maintaining several versions of the same file in different folders
is a bad idea, because at some point their content gets out of sync.
This is exactly what happened to 'respawn.sh': sleep()ing was only
implemented in 'osmo-bts-master/respawn.sh', other versions of this
file would simply ignore '$SLEEP_BEFORE_RESPAWN'.

The easiest solution would be to have all common files in a single
directory, however Docker does not allow to ADD files from outside
of the build context.  In other words, all files must be in the
same directory as the Dockerfile itself.

Modify 'make/Makefile' in order to copy the contents of common
directory to the current build context ('pre-build' target) and
remove it after building ('post-build' target).

Change-Id: I3ec86c8610b3b43d39ea8e3da444861d317ced4e

Share a single instance of Release.key between all images

This change employs the same approach as [1] for Release.key.

Change-Id: Ic51d6e6557ee0b56d21d8ff4feaf04bc02e41a2e
Related: [1] I3ec86c8610b3b43d39ea8e3da444861d317ced4e