[git submodule test] u-boot-arndale
[test] replace u-boot to u-boot-arndale
[U-BOOT-ARNDALE] support u-boot in secure mode on arndale board

- .gitmodules: u-boot-arndale url
- README.md: add "Testing u-boot on ARNDALE board using git submodule"
Merge branch 'arndale' of https://github.com/Wonseok/khypervisor into proto2-temp
merge [U-BOOT-ARNDALE]