boards: nxp: mcx_n9xx_evk_cpu0_qspi: boot directly from QSPI flash

Configures QSPI board variant to boot directly from external QSPI flash
on the FlexSPI.  Secondary bootloader is no longer required.  Moves the
MCUboot boot_partition to QSPI flash.

Signed-off-by: Derek Snell <derek.snell@nxp.com>