Sunxi current a20 dev

This pull adds:
* support for the Marsboard A20 version
* code to power-on and keep the second A20 core in a wait loop
* modified smp-boot API (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.

Updated SMP boot protocol
* check if CPU1 is held in reset
* if in reset, normal power-on
* if not in reset, set the boot_addr and perform a "sev" OR a write of (2) to cpucfg->event_in

It also includes some miscellaneous fixes:
* Add ELF loading support when networking is selected
* Cleanup hacks in cmd_elf.c
* Make default prompts a little cleaner
* Set and change the default loadaddr to support loading large images
* Remove ZFS filesystem - 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