Move stack to SDRAM as specified in memory map.

* After initializing the page table and enabling MMU,
  the pre-MMU stack becomes invalid leading to a fault.
  This was fixed by moving the stack to SDRAM as specified
  in LOADER_MEMORYMAP before ARM entry point start_netbsd.

Signed-off-by: Ithamar R. Adema <ithamar@upgrade-android.com>
Corrected check to ensure that command line options are indeed passed.

* Currently, no command line options are being passed via u-boot
  to haiku. However, the comparison doesn't ensure that cmdline
  is not an empty string - it merely ensures cmdline is not null.

Signed-off-by: Ithamar R. Adema <ithamar@upgrade-android.com>