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/scripts/imx6_app.py

@@ -55,7 +55,9 @@ def _env_lookup(key, default=None):
 REPO = r"E:\Xilinx_design\smod\SMOD_ATSC\my_exciter"
 APP_SUBDIR = "imx6/src/atsc_app"
 
-VM_HOST = _env_lookup("CROSS_VM_HOST", "192.168.223.128")
+# No default: the VM's IP moves with the VMware segment, so a stale fallback would
+# silently connect to the wrong host (or hang). Fail loudly instead.
+VM_HOST = _env_lookup("CROSS_VM_HOST")
 VM_USER = _env_lookup("CROSS_VM_USER", "bowei")
 VM_PASS = _env_lookup("CROSS_VM_PASS")
 VM_BUILD_DIR = "atsc_head_build"  # relative to the VM home dir