scripts: only set CROSS_COMPILE when unset to begin with

Don't mess with CROSS_COMPILE if it's already set up in the user's
environment. This allows the user to specify the cross compiler that
they wish to use.

Change-Id: I15873f79a30e1da91ee34eb31c713e5cd767c372
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>