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.

review third-party license tags

Made third-party license tags more precise.

RISCV: Added stdout-path to hifive dts

This sets the first UART as the stdout-path dts setting in the
"chosen" node in the HiFive device tree.

Change-Id: Icb6b9abf999bdd8a278df5a2ba73ad492af06a24

RISCV: Add Hifive unleashed platform

This change adds support for Hifive unleashed board. It also removes the
outdated hifive suport from the spike platform.