Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

am335x: use correct boneblack memory size

Overlay the incorrect memory size in the device tree, and enlarge the
kernel window appropriately.
BeagleBone Black has 512MiB memory.