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.

Style: remove stray tabs

style: consistently attach return type

Add attach-return-type to astyle

serial-input: Reduced duplicate imx drivers into one file

Moved imx6q-uart.c and imx31-uart.c -> imx.c and updated config.cmake
appropriately.