When building projects from COHT repository, there are 3 ways to provide
the value of "REPO_PARENT" to the project sub-drectories.

1) Define default value of REPO_PARENT in each project's Makefile. This
can be harmful when building the project outside COHT surroundings.

2) Pass the value of REPO_PARENT to each projct during build. This will
need lots of additional code, because each project has its own
sub-directories with separate Makefiles.

3) Export the value as an environment variable so that it is visible to
all.

Thus, we export the value.

Signed-off-by: Vaibhav Gupta <vaibhav.gupta@cern.ch>