DCENT_OS am3-bb-s19jpro SD-card payload

Status: native AM3 BB mining SD-card payload.

This directory is intentionally not a NAND flasher. Add verified AM335x boot
artifacts (MLO, u-boot.img, uImage, and a carrier-aware DTB) from a lab-approved
stock restore bundle, then write a raw SD image outside this post-image hook.
The DTB must contain an S19J_IO_BOARD or am335x-boneblack-btm identity marker;
the checked-in am335x-s19jpro.dts is legacy reference material and is not a
build input. Do not finalize AM335x media with Windows format/copy; MLO must
remain the first real FAT file entry.

Boot media must use ramdisk.gz, the U-Boot legacy ramdisk wrapper around the
raw CPIO. uramdisk.image.gz is kept for inspection and recovery only.
No ext2 rootfs is part of the AM3-BB SD-first boot path.

NAND install/revert is disabled until SD recovery, full NAND backup, and
restore-to-stock are proven on this exact AM335x BB lane.

Cold boot starts dcentrald with --am3-bb-mining, rescue SSH enabled, and MCP
on localhost:3000. ASIC UART mining uses kernel ttyS1/ttyS2/ttyS4 at 115200 by
default; FastUART remains lab-only. DO NOT include uart_trans.ko, stock cgminer,
monitor-ipsig, or daemons in the SD payload.
