libsel4: fix license tags

These files should have been released under BSD-2-Clause in the first
place (as per parent LICENSE.md file).

Closes #245

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

Enabled IPC debug features under new config

Introduced a new config flag to enable
userError format strings to be written to the IPC buffer.
Another config bool has been introduced to toggle
printing the error out and this can also be set at runtime.

Signed-off-by: Saer Debel <saer.debel@data61.csiro.au>

replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

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.

Remove platform IPC buffer register.

This removes the assumption that each platform sotres the IPC buffer
address in a platform-specific register. The IPC buffer address is
instead stored in a thread-local variable in libsel4 which must be
initialised by the runtime.