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/cbtable.c b/cbtable.c
index 3e97ed7..6a6f7a9 100644
--- a/cbtable.c
+++ b/cbtable.c
@@ -22,6 +22,8 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
  */
 
+#include <unistd.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <sys/types.h>
 #include <string.h>