soc: arm: nxp_imx: rt: refactor flexspi boot config

* Move definition of flexspi_nor_config_t into soc/ dir so it can
  be shared by all i.MX RT based boards.
* Use Kconfig symbol CONFIG_NXP_IMX_RT_BOOT_HEADER instead of
  HAL define (which is set based on the Kconfig symbol)
* Rename board files to flexspi_nor_config.c since they
  are already namespaced by the board dir.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>