build/core: Use better default compiler flags

Use -O3 for the compiler, -O2 for the linker

Change-Id: Ib9c3cd6d6ddf16c8bd20149a051ff2c17491a29d
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>

Conflicts:
	core/combo/select.mk