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/references/imx6.md

@@ -17,7 +17,8 @@
 
 ## 交叉编译
 
-工具链在 VM `bowei@192.168.223.128`（密码见 `~\skills_env.md` 的 `CROSS_VM_PASS`）：
+工具链在 Ubuntu VM 上（主机/用户/密码见 `~\skills_env.md` 的 `CROSS_VM_HOST` /
+`CROSS_VM_USER` / `CROSS_VM_PASS`——IP 随 VMware 网段变动，不要写死）：
 
 ```
 /opt/fsl-imx-x11/4.1.15-2.0.0/environment-setup-cortexa7hf-neon-poky-linux-gnueabi