Set default USE flags values as local variables.

Setting variables such as BRILLO_USE_FOO can leak to other makefiles,
so the local default value set by another project can affect the
default behavior. This patch defines local variables with the local
default value for the USE flags.

Bug: None
TEST=mmma external/libbrillo

Change-Id: Iefca0c4de48d7bd09d41b06ca18f1c95716f6fce
1 file changed