big: Create a nyan_big mainboard which is a copy of nyan.

big: Create a nyan_big mainboard which is a copy of nyan.

The nyan_big mainboard is very similar to nyan, but will be different in a few
ways. For instance, the BCT will be different, and the GPIOs may need to be
configured slightly differently.

This change also adds prefixes to the kconfig variables in "choice" blocks
for both boards since having multiple instances of choice blocks with the same
options confuses kconfig even if all of the instances have mutually exclusive
dependencies.

Change-Id: I290a32e47fc118bd4b86d543df617ad324325dbc
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://chromium-review.googlesource.com/183532
Reviewed-by: Tom Warren <twarren@nvidia.com>
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
(cherry picked from commit d1a453fe1aa68b3d12936dd48cc6c94b54f81579)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6927
Tested-by: build bot (Jenkins)