Reverse IS_EXTERNAL check, and guard libedgetpu

- Seems to me like these checks were backwards: without IS_EXTERNAL set,
edgetpu-api is no longer a valid build target.
- Also, add libedgetpu to the guarded set that won't work externally.

Change-Id: I3d52e5703bec5426b705f9dd77e78f260ae8b4c3
(cherry picked from commit edcbf85401cfec784635680d7a4dd19cbc726d66)
2 files changed