[list] Add list_first_entry()

There are several points in the iPXE codebase where
list_for_each_entry() is (ab)used to extract only the first entry from
a list.  Add a macro list_first_entry() to make this code easier to
read.

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

src/include/ipxe/list.h

@@ -157,6 +157,19 @@ static inline int list_empty ( const struct list_head *list ) {
 	list_check ( (list) );				\
 	container_of ( list, type, member ); } )
 
+/**
+ * Get the container of the first entry in a list
+ *
+ * @v list		List head
+ * @v type		Containing type
+ * @v member		Name of list field within containing type
+ * @ret first		First list entry, or NULL
+ */
+#define list_first_entry( list, type, member )		\
+	( list_empty ( (list) ) ?			\
+	  ( type * ) NULL :				\
+	  list_entry ( (list)->next, type, member ) )
+
 /**
  * Iterate over entries in a list
  *