or1k: set heap start for optimsoc-gzll

or1k: set heap start for optimsoc-gzll

- With the gzll kernel we have two different loading options:

  - If the image is loaded to the global memory, the bootstrapping
    loads the kernel to local memory. Applications are loaded on
    demand. The heap then starts right after bss.

  - If the image is pre-loaded to the local memory it includes the
    application binaries right after bss. The heap then starts after
    the application objects.

- We can check if this is a gzll kernel as it has the string "gzll" at
  0x2000. At 0x200c we then find the end of the application objects in
  the image. If there is no global memory we set _or1k_heap_start to
  this value.

    * or1k/boards/optimsoc.S: Heap for gzll kernel