build/core: Add linaro_compilerchecks.mk

Add linaro_compilerchecks.mk, allowing to run some checks on the
compiler version and supported flags before setting them

Change-Id: I96e1277e74c0ea3457677bef39644aaece01f653
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
build/core: Include linaro_compilerchecks.mk

Include linaro_compilerchecks.mk in core/config.mk to allow the macros
to be used in board configs

Change-Id: I3a0dfd0a3a1c09961e43ef8c2f3cbe50b8c00e20
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
build/core: Use better default compiler flags

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

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>