EXTERNAL_WARNINGS_NOT_ERRORS -> ENABLE_WERROR and be "TRUE"/""

It's easier to type 'make ENABLE_WERROR= <module>' if one wants that.

Change-Id: I2bb9911259f41ecae27dc110723f3364b3ff09cf
Rename functions to avoid name collisions more generally.

Luckily, in this case it's easy to find the only place where we load the symbol.

Change-Id: I224fd1e08f268095f8be58b089a643d434bc7a98