core: tasks: kernel: fix selection of 4.9-linaro toolchain

Change-Id: Ib156d09fd7063d99775f4e4f7df0d9a0d11a015f
core: eos_config: switch optimization level to -O2 by default

-Os optimizes for size, which produces smaller executables but
significant performance constraints (revealed in benchmarks).

Thus, switch the optimization level back to -O2 by default,
but set -Os for targets with a small system partition (e.g. Xoom).

Change-Id: Ibda409382524906d90dd3d9537a024f4fde6be5b