Added b2g specific valgrind and vex repos
Bug 854517 - Add b2g specific valgrind and vex repos (for manifests missed in last push)
Reverting valgrind/vex commits (Bug 854517 e16fe0ebb94ef527bea4876b4ef994fb4c577d8a e819e3792dfe2b2f434261b2e47e2279830de45f) due to emulator bustage
Bug 852583: switch emulator system/core and qemu-kernel to B2G github.

Change-Id: Ibc1989882c27f1fb790474e65ebb7333c93186ec