[malloc] Allow heap to specify block and pointer alignments

Size-tracked pointers allocated via umalloc() have historically been
aligned to a page boundary, as have the edges of the hidden memory
region covering the external heap.

Allow the block and size-tracked pointer alignments to be specified as
heap configuration parameters.

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

src/include/ipxe/malloc.h

@@ -45,6 +45,11 @@ struct heap {
 	/** List of free memory blocks */
 	struct list_head blocks;
 
+	/** Alignment for free memory blocks */
+	size_t align;
+	/** Alignment for size-tracked allocations */
+	size_t ptr_align;
+
 	/** Total amount of free memory */
 	size_t freemem;
 	/** Total amount of used memory */