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).
