utils.sh: tidy up the script

Using "tf-a-<TF-A_LTS_BRANCH>" to change the download space
for the LTS branch

Change-Id: I4d6e7ff1f0ab406ef83e77f4bea4ca8fbdb1e3b8
Signed-off-by: Arthur She <arthur.she@linaro.org>
(cherry picked from commit 2c9ca72a1f20d8ce437943b7e6f0f8c9b987ed09)
5 files changed