Add symlink from gpl-2.0.txt to a COPYING file

While it is good that the "Licenses/" directory contains the text
for all licenses that are applicable to u-boot code, it is harder
to determine at a glance what the default and/or preferred license
is. While humans can look at the Licenses/README file, this is not
machine parseable, making it tricky for license detection tools to
automatically determine/report on the overall / aggregate u-boot
license.

The project previously had a top level COPYING file containing a
short blurb, followed by the GPL license text. This was removed
back in commit eca3aeb352c964bdb28b8e191d6326370245e03f when
the "Licenses/" directory was introduced. For the benefit of
automated tools, it is helpful to retain a top level COPYING
file in the repository. Rather than duplicate the license text,
however, a symlink from the Licenses/gpl-2.0.txt file should
suffice.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reported-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Tom Rini <trini@konsulko.com>

diff --git a/COPYING b/COPYING
new file mode 120000 (symlink)
index 0000000..1ed3ba6
--- /dev/null
+++ b/COPYING
@@ -0,0 +1 @@
+Licenses/gpl-2.0.txt
\ No newline at end of file