cbfstool: provide structure to linux payload builder

This change started with tracking down a bug where the trampoline
size was not being taken into account for sizing the output buffer
leading to a heap corruption.  I was having a hard time keeping
track of what num_segments actually tracked as well as what parts
were being placed in the output buffer. Here's my attempt at
hopefully providing more clarity.

This change doesn't crash when adding a bzImage:
$ dd if=/dev/zero of=bb.bin bs=64 count=1
$ ./cbfstool tmp.rom create -s 4M -B bb.bin -m x86 -a 64
$ ./cbfstool tmp.rom add-payload -f ~/Downloads/bzImage -C "1" -n
"fallback"/payload

Change-Id: Ib1de1ddfec3c7102facffc5815c52b340fcdc628
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5408
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

util/cbfstool/common.h

@@ -63,6 +63,15 @@ static inline void buffer_set_size(struct buffer *b, size_t size)
 	b->size = size;
 }
 
+/* Initialize a buffer with the given constraints. */
+static inline void buffer_init(struct buffer *b, char *name, void *data,
+                               size_t size)
+{
+	b->name = name;
+	b->data = data;
+	b->size = size;
+}
+
 /*
  * Splice a buffer into another buffer. If size is zero the entire buffer
  * is spliced while if size is non-zero the buffer is spliced starting at
@@ -71,9 +80,7 @@ static inline void buffer_set_size(struct buffer *b, size_t size)
 static inline void buffer_splice(struct buffer *dest, const struct buffer *src,
                                  size_t offset, size_t size)
 {
-	dest->name = src->name;
-	dest->data = src->data;
-	dest->size = src->size;
+	buffer_init(dest, src->name, src->data, src->size);
 	if (size != 0) {
 		dest->data += offset;
 		buffer_set_size(dest, size);