u-boot: Add nyan big chromebook configs

This adds U-Boot configuration for the Acer Chromebook 13 (CB5-311,
C810), also known as "nyan_big" in the U-Boot upstream defconfigs. Also
adds a shared "nyan" board directory to share with others having the
same baseboard. The config is almost the same as upstream defconfig, but
with REMAKE_ELF and POSITION_INDEPENDENT enabled.

Untested since I don't have the nyan big chromebook.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>