[image] Add the concept of a static image

Not all images are allocated via alloc_image().  For example: embedded
images, the static images created to hold a runtime command line, and
the images used by unit tests are all static structures.

Using image_set_cmdline() (via e.g. the "imgargs" command) to set the
command-line arguments of a static image will succeed but will leak
memory, since nothing will ever free the allocated command line.
There are no code paths that can lead to calling image_set_len() on a
static image, but there is no safety check against future code paths
attempting this.

Define a flag IMAGE_STATIC to mark an image as statically allocated,
generalise free_image() to also handle freeing dynamically allocated
portions of static images (such as the command line), and expose
free_image() for use by static images.

Define a related flag IMAGE_STATIC_NAME to mark the name as statically
allocated.  Allow a statically allocated name to be replaced with a
dynamically allocated name since this is a potentially valid use case
(e.g. if "imgdecrypt --name <name>" is used on an embedded image).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/embedded.c

@@ -31,8 +31,9 @@ EMBED_ALL
 /* Image structures for all embedded images */
 #undef EMBED
 #define EMBED( _index, _path, _name ) {					\
-	.refcnt = REF_INIT ( ref_no_free ),				\
+	.refcnt = REF_INIT ( free_image ),				\
 	.name = _name,							\
+	.flags = ( IMAGE_STATIC | IMAGE_STATIC_NAME ),			\
 	.data = ( userptr_t ) ( embedded_image_ ## _index ## _data ),	\
 	.len = ( size_t ) embedded_image_ ## _index ## _len,		\
 },