fileio: Unify whole-file reads

We do whole-file reads in a few places, so unify to a fileio_read_file()
function.

To do this, we change the type of struct image->buf to a uint8_t *.
Where we do pointer manipulation on the image buffer, we need a
temporary void * variable.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>

image.h

@@ -48,8 +48,7 @@ struct region {
 };
 
 struct image {
-	int		fd;
-	void		*buf;
+	uint8_t		*buf;
 	size_t		size;
 
 	/* Pointers to interesting parts of the image */