imx6: document VM bootstrap, stop hardcoding the cross-compile VM's IP

The VMware segment changed, so the VM moved from 192.168.223.128 to
192.168.154.128 and every hardcoded reference was stale. A freshly-installed
VM also has neither sshd nor the toolchain, which previously only surfaced
half-way through a modify_imx6 iteration.

- fpga-imx6-flow: new "VM prerequisite check" section -- how to detect a
  missing sshd (Connection refused) and reach the guest over vmrun/VMware
  Tools when SSH is impossible, plus the Yocto SDK install + ARM-ELF
  verification step. Records the real gotchas hit during install:
  unattended-upgrades holding the dpkg lock, EOL xenial repos, and CRLF
  breaking scripts pushed into the guest.
- smod_atsc_dev SKILL/imx6.md: refer to CROSS_VM_HOST/USER/PASS, no literal IP.
- imx6_app.py: drop the stale 192.168.223.128 default so a missing
  CROSS_VM_HOST fails loudly instead of dialing the wrong host.

Machine-local values (IP, credentials, vmx/vmrun paths, toolchain prefix)
live in ~/skills_env.md, which stays out of git.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

smod_atsc_dev/SKILL.md

@@ -32,7 +32,7 @@ FPGA 做 8VSB 调制 / DPD / AGC / TS 前处理，imx6 跑 `atsc_app`（配置 A
 | Vivado | 见 `~\skills_env.md` 的 `VIVADO_ROOT`（其下 `bin\vivado.bat`）——路径因机器而异，不要写死盘符 |
 | imx6 源码 | `my_exciter\imx6\src\atsc_app`（Makefile 在该目录根） |
 | imx6 设备 | `192.168.1.143`，用户 `root`，密码见 `~\skills_env.md` 的 `SMOD_SSH_PASS`（网页 + SSH；Windows 主机 `192.168.1.125` 同网段） |
-| 交叉编译 VM | `bowei@192.168.223.128`，密码见 `~\skills_env.md` 的 `CROSS_VM_PASS`，构建目录 `~/atsc_head_build` |
+| 交叉编译 VM | 主机/用户/密码见 `~\skills_env.md` 的 `CROSS_VM_HOST` / `CROSS_VM_USER` / `CROSS_VM_PASS`（IP 会变，不要写死），构建目录 `~/atsc_head_build` |
 | 工具链 | `/opt/fsl-imx-x11/4.1.15-2.0.0/environment-setup-cortexa7hf-neon-poky-linux-gnueabi` |
 | app 部署路径 | `/aw/anywave/app/imx_atsc_app`（先写 `imx_atsc_app_head`，再 `cp -a` 覆盖 active） |
 | app 日志 | `/var/module_log/imx_atsc_app.log` |