Reverting manifest revert now that valgrind repos are fixed.

Revert "Reverting valgrind/vex commits (Bug 854517 e16fe0ebb94ef527bea4876b4ef994fb4c577d8a e819e3792dfe2b2f434261b2e47e2279830de45f) due to emulator bustage"

This reverts commit 0931407d1b2689fb8d08ba25683ab558bf47d920.
And reverted again, because vex symlink checkout doesn't work on any other box

Revert "Reverting manifest revert now that valgrind repos are fixed."

This reverts commit 09429853a96a23b8c7f8c8fee74b4500a34d2e49.
Bug 854517: Add external/valgrind and external/VEX to manifests
Bug 852583: switch emulator system/core and qemu-kernel to B2G github.

Change-Id: Ibc1989882c27f1fb790474e65ebb7333c93186ec