build: rename CONFIG_FLAGS to CONFIG_CFLAGS

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
build: add compilation option CONFIG_LTTNG

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
documentation: add documentation for CONFIG_LTTNG

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add compilation option CONFIG_MPI=n to disable MPI

It is useful to be able to use the code without needing MPI.

Requested-by: George K. Thiruvathukal <gkt@cs.luc.edu>
Link: https://github.com/GeneAssembly/biosal/issues/555
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_transport: add fallback transport that does nothing (mock)

When both PAMI and MPI are disabled, Thorium still needs to know
the number of ranks and the rank number from the transport
subsystem. The mock_transport can not transport anything and it just
rank to 0 and size to 1.

Link: https://github.com/GeneAssembly/biosal/issues/555
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>