script/gcc: Fix the Linaro variation condition.

$linaro_version is always not empty. Check if ${CT_CC_VERSION}  starts with "linearo-" to determine the download location.

Signed-off-by: Elson Wei <elson.wei@gmail.com>
I'm actually going to reject this change.
linaro_version and linaro_series need to be set to "" before being set by the above expressions.
That was my fault in the preceding change that caused this issue.
Also, the same needs to be done for gdb.

But, thank you for this!