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/chipset_enable.c b/chipset_enable.c
index 04b1a1c..9356b1e 100644
--- a/chipset_enable.c
+++ b/chipset_enable.c
@@ -27,6 +27,7 @@
 
 #define _LARGEFILE64_SOURCE
 
+#include <unistd.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>