Change PLATFORM_ROOT to TF_PLATFORM_ROOT
...
Since we use "?=" to set PLATFORM_ROOT, it is better to change the
name to be more special, or else it may be overridden by some
environment variables, such as in some CI build environments.
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Heyi Guo
committed
on 5 Mar 2018