Update the way we use Coccinelle to find out-of-style code that
need to be modernised.
* sg/coccicheck-updates:
coccinelle: extract dedicated make target to clean Coccinelle's results
coccinelle: put sane filenames into output patches
coccinelle: exclude sha1dc source files from static analysis
coccinelle: use $(addsuffix) in 'coccicheck' make target
coccinelle: mark the 'coccicheck' make target as .PHONY