Use SPDX license identifiers ...

To make software license auditing simpler, use SPDX[0] license
identifiers instead of duplicating the license text in every file.

NOTE: Files that have been imported by FreeBSD have not been modified.

[0]: https://spdx.org/

Change-Id: I80a00e1f641b8cc075ca5a95b10607ed9ed8761a
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

Fix incorrect copyright notices ...

Some files have incorrect copyright notices, this patch fixes all
files with deviations from the standard notice.

Change-Id: I66b73e78a50a235acb55f1e2ec2052a42c0570d2
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

Make:Use environment variables for OS detection. ...

Add make helper files to select the appropriate settings for the build
environment. Selection is made in make_helpers/build_env.mk, which
selects other files to include using generic build environment settings.
The Trusted Firmware Makefile and supporting tool Makefiles are updated
to include build_env.mk instead of unix.mk.

NOTE: This change does not fully enable builds in other build
      environments. It facilitates this without compromising the
      existing build environments.

Change-Id: Ic4064ffe6ce158bbd16d7cc9f27dd4655a3580f6