Remove unneeded #include statements completely

Unistd.h was only used to get a definition of NULL in all files. Add our
own NULL #define and remove unistd.h from flash.h
stdio.h has no place in flash.h, it should be included only in files
which really need it.
Add #include statements in individual .c files where needed.

Replace a few printf with msg_* to eliminate the need for stdio.h.

Corresponding to flashrom svn r1021.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>
diff --git a/cli_classic.c b/cli_classic.c
index baca900..f541084 100644
--- a/cli_classic.c
+++ b/cli_classic.c
@@ -21,6 +21,7 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
  */
 
+#include <stdio.h>
 #include <fcntl.h>
 #include <sys/types.h>
 #include <sys/stat.h>