v1.6.4

Fix eMMC erlinit.config always using SD device paths (mmcblk1 instead of
mmcblk0). Moved erlinit.config out of rootfs_overlay/ into variant-specific
overlays (rootfs_overlay_sd/, rootfs_overlay_emmc/) so that
Nerves.Erlinit.system_config_file/1 no longer overwrites the correct
Buildroot-applied config during mix firmware.