Add manifest groups.

This makes it possible to download fewer of the projects.

Groups currently supported:
-device: projects that are specific to flagship devices.
-darwin: tools are that only used on darwin hosts.
-linux: tools that are only used on linux hosts.
-arm: tools that are only used for ARM targets.
-mips: tools that are only used for MIPS targets.
-x86: tools that are only used for IA targets.

Change-Id: I8ed1229a7306bd465a4cb7cb992819e720c88769
1 file changed