boards: nxp: change definitions to local scope

1.'XIP_EXTERNAL_FLASH' and 'USE_HYPERRAM' are only used in
zephyr/boards/nxp/mimxrt1180_evk/xip/evkmimxrt1180_flexspi_nor_config.c,
so just change them to local scope.

2.'XIP_BOOT_HEADER_XMCD_ENABLE' is only used in
zephyr/boards/nxp/mimxrt1170_evk/xmcd/xmcd.c,
so just change it to local scope.

Signed-off-by: Zhaoxiang Jin <Zhaoxiang.Jin_1@nxp.com>