asyncdispatch on RTOS(Flat Memory)

[takekikuchi]

It seems that gDisp is placed only one place in the BSS area, probably due to 
flat binary.
    
    
      3 .bss          0001efa0  22086000  22086000  00085f20  2**8
                      ALLOC
    
    $ cat nuttx.map |grep gDisp
     .bss.gDisp__pureZasyncdispatch_u2601
                    0x00000000220a04f4                
gDisp__pureZasyncdispatch_u2601
    
    
    Run

Since NuttX has a function to dynamically load elf-format binaries, it may be 
necessary to consider using this function.