Sunxi current marsboard a20

This adds support for the Marsboard A20 version.

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