Integrate u-boot-seco-imx/seco_lf_v2021.04-lvds_cha_wvga

Commit: clea-os/bsp/nxp/u-boot-seco-imx@df52b8a7

[D18][BOARD] improve U-Boot video environment setup

Video link on only if specific fdt video overlay are selected

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Commit: clea-os/bsp/nxp/u-boot-seco-imx@3dc29dd6

[D18][ENV] add LVDS CHA WVGA overlay option

Also, change multi-video overlay handling: use intercompatible video overlays instead of single combined ones.