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>

resources/u-boot/nyan/board.cfg

@@ -0,0 +1,3 @@
+ubtree="nyan"
+ubrevision="e092e3250270a1016c877da7bdd9384f14b1321e" # v2022.07
+arch="ARMv7"

resources/u-boot/nyan_big/board.cfg

@@ -0,0 +1,2 @@
+ubtree="veyron"
+arch="ARMv7"