ANDROID: Support building with build.sh

Add a build config to build U-Boot for crosvm. Use it like this:

$ BUILD_CONFIG=u-boot/build.config.crosvm.x86_64 build/build.sh

Bug: 162801433
Change-Id: Ibc67545659b32d1dcc5920ef08cfca2abe27c0eb
2 files changed