* by default, snapshot will be saved every 10s

* now mainargs is supported by nexus-am

* To run more programs on debian, we use 256MB memory described in dts.
  But if the hardware still provides 128MB memory, the memory access
  will wrap around when Linux allocate a physical page excceed 128MB,
  which causes wrong memory access.

* also put mainargs at 0x80001000 to match AM

* now we can pass the image used in AXI4RAM as an argument of emu