portage: add wrappers/helpers for interacting with portage

This creates emerge/ebuild/portageq wrappers people can execute directly
(as well as the android build system will use), and the target-specific
config files they rely on.

The tools should work off of the android env vars to locate the current
board/build directories rather than hardcoding any of them in the output.

TEST=built packages and they worked in qemu

Change-Id: I97171f3d8d88230880af9ba3045d691951e0b5fe
5 files changed
tree: d178fcfc3daa3a9033a14f6c93df5160604c0c23
  1. portage/