Create csr_build gadget to operate in pre_build phase.

Still needs completedCallback to work properly.
Use managePool for bunches of non-interactive jobs.
Fix comment.
Merge branch 'devel' into csr3
Remove debugging messages.
Add a .csr3_stdout file, raise GadgetFailed on failure.
Add and use GadgetCommand class.
Change description of create_cmds
Write fsdb.sh file without using utils.open.