soc: nxp_kinetis: Make kinetis flash configuration field configurable

Kinetis SoCs have a 16-byte flash configuration field that must be
loaded at a specific address in flash. This field is not needed if we
are building an image to be chainloaded by MCUboot or a RAM-only image,
so we can exlude it in these cases and recover some wasted flash.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>