Sunxi current a20 dev3

(Third time lucky)
This pull adds:

* support for the Marsboard A20 version.
* code to power-on and keep the second A20 core in a wait loop.
* with secondary core in a wait loop, this allows a modified SMP boot protocol in Linux.
  * This is backwards compatible with existing Linux code.
  * TODO: update Linux to support it. This has been tested with a different (non-Linux) SMP kernel.
  * Secondary power-on is important for debugging - JTAG tools require all cores you wish to debug to be powered-on.

Improved, backwards compatible Linux sun7i SMP boot protocol:
* check if CPU1 is held in reset
* if in reset, normal power-on sequence
* if not in reset, set the cpucfg->BOOT_ADD register AND
* perform a "sev" OR a write of (2) to cpucfg->EVENT_IN register

Pull also includes some miscellaneous fixes:

* Add ELF loading support when networking is selected
* Make default sun7i prompts a little cleaner
* Remove ZFS filesystem config - It's not very common, if you really need it, I suggest adding it to the boards.cfg or edit .config manually
* minor typo fixes