adb: Don't force -O2

Don't force -O2 over -O3 -- the O2 hardcode is there to force
optimizations, not to reduce them...

Change-Id: Ic75eeb767db4926f519580fba8f5f7b8e593df4f
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>