build: Add support for TARGET_EXTRA_C{,PP}FLAGS

Add support for setting TARGET_EXTRA_C{,PP}FLAGS in board configs.
This can be used to set compiler flags for a board that break other
boards, such as -march=cortex-a9 or -DUSE_CUSTOM_EXTENSIONS_FOR_XYZ
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>