Allow globally disabling the relocation packer.

The relocation packer is part of prebuilts/misc, and pulling that
repository into the manifest requires pulling in a lot of other things
not needed by the simpler builds (like the clang toolchain manifest).

Bug: http://b/17441393
Change-Id: If4a94804fc1a3f81215b840247f8e332d0b510c1
2 files changed