coccicheck: make SPFLAGS more useful

SPFLAGS is set early, it means that any heuristics done on
coccicheck cannot be overridden currently. Move SPFLAGS
after OPTIONS and set this at the end. This lets you override
any heuristics as coccinelle treats conflicts by only listening
to the last option that makes sense.

v3: this patch was added in the v3 series
v4: Update Documentation/coccinelle.txt explaining how
    SPFLAGS works as well.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Acked-by: Nicolas Palix <nicolas.palix@imag.fr>
Signed-off-by: Michal Marek <mmarek@suse.com>