Add support for the C variable in the coccicheck script

This patch makes it possible to use the Coccinelle checker with the C
variable of the build system. To check only newly edited code, the
following command may be used:

'make C={1,2} CHECK="scripts/coccicheck"'

This runs every semantic patch in scripts/coccinelle by default. The
COCCI variable may additionally be used to only apply a single
semantic patch.

Signed-off-by: Nicolas Palix <npalix@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Michal Marek <mmarek@suse.cz>