TEST-ONLY: cros: exynos: Change U-Boot load address

This is intended for developers wanting to use chained U-Boot to
cherry-pick.  It works around the fact that the built-in U-Boot has
<http://crosbg.com/37195>.

Change-Id: Ibe4be8632c23aaa4e1e58a8e89bbc9e9d923e197