In preparation to support other A20 based boards, rename the CUBIEBOARD2
kernel configuration to A20.

There are other boards (namely the banana pi) that use exactly the same
devices.

Additionally, we are moving from static FDT support (DTB compiled
in-kernel) to DTB passed to kernel by the boot loader (ubldr). The u-boot
for these boards are already available on ports and as the crochet support
for these boards isn't committed yet, this should not bring any issues.

Discussed with: ian