Tear out old heap code, replace with code that simply allocates memory

for use by malloc().

This breaks the image-loading code (which previously used the heap to
allocate the buffer for downloading the image), but that's not a major
concern since I'm going to tear out all the image formats within the next
couple of days anyway.  Byebye, NBI!  :)

src/include/gpxe/heap.h

@@ -0,0 +1,15 @@
+#ifndef _GPXE_HEAP_H
+#define _GPXE_HEAP_H
+
+/**
+ * @file
+ *
+ * Heap
+ *
+ */
+
+extern char heap[];
+
+extern void init_heap ( void );
+
+#endif /* _GPXE_HEAP_H */