imx8mm: imx8mm_evk: fix BOOTSTD boot

Select BOOTSTD_FULL and BOOTSTD_BOOTCOMMAND
Correct DEFAULT_FDT_FILE
Correct env file for imx8mm_evk_fspi_defconfig

Fixes: 364ba68ed1a ("imx: imx8mm_evk: Switch to BOOTSTD")
Reported-by: Ludwig Nussel <ludwig.nussel@siemens.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
3 files changed