cmd: Implement bootselect
Mostly copied from NXP bootloader, supports ENV mode and GPIO button mode. Bootscript changes: seco-ne/yocto/layers/meta-seconorth-mtk!59 (merged)
Mostly copied from NXP bootloader, supports ENV mode and GPIO button mode. Bootscript changes: seco-ne/yocto/layers/meta-seconorth-mtk!59 (merged)