boot - Put loader heap in high memory

boot - Put loader heap in high memory

* We ran out of low BIOS memory.  The initrd.img.gz unpacking requires a
  bit more than ~32KB of temporary space and it ran the loader out of
  low-BIOS heap memory.

  This can cause the loader to fail to load the image and prevent proper
  crypto or single-user bootstrapping.

* Steal the heap from high-memory, just ender the kernel+modules load area
  limit.  For now, steal 1MB, which gives the loader plenty of space again.
  The kernel will lose 1MB of physical memory, boo-hoo.

* The loader must remove the heap from the SMAP passed to the kernel.

* Move any objects requiring VTOPSEG/VTOPOFF out of malloc and into a
  static or stack declaration.

Reported-by: cgag, multiple