Only set -Wall once.

We set -Wall in the global cflags, and then again at the point where we
decide whether or not to add -Werror. The trouble with this is that it
undoes the effect of any attempt to disable a warning implied by -Wall.

Discovered while trying to enable -Wmisleading-indentation (which is
part of -Wall) in a way that doesn't apply to external/ or vendor/.

Test: treehugger
Change-Id: I68d74fb05922dd9f6bd4c8423ca69b485c15e3d2