docs: Add CRITICAL note about ACCEPT_FSL_EULA in local.conf

The firmware-imx recipe checks local.conf for ACCEPT_FSL_EULA, not
just the environment variable. Without this, air-gapped builds fail
at task 1980 with EULA error.

Also verified: repo is public (Private: False via API check).

Fixes: Add 'ACCEPT_FSL_EULA = "1"' to local.conf in both:
- Step 3 (Initialize Build Environment)
- Step 7 (Air-Gapped Deployment)

docs/02-mirror-creation/step-by-step-mirror-creation.md

@@ -125,6 +125,13 @@ export MACHINE=tqma6ul-multi-mba6ulx
 > The `ACCEPT_FSL_EULA=1` environment variable auto-accepts it.
 > Without this variable, the setup script blocks waiting for input.
 
+**CRITICAL:** Some recipes (firmware-imx) check local.conf, not just the
+environment variable. Add this to your local.conf:
+
+```bash
+echo 'ACCEPT_FSL_EULA = "1"' >> conf/local.conf
+```
+
 **Verify:**
 ```bash
 bitbake -e | grep -E "^MACHINE=|^DISTRO=|^DL_DIR="
@@ -256,6 +263,9 @@ EOF
 # Build
 cd sources/ci-meta-tq
 export ACCEPT_FSL_EULA=1
+
+# CRITICAL: Also add to local.conf
+echo 'ACCEPT_FSL_EULA = "1"' >> conf/local.conf
 export MACHINE=tqma6ul-multi-mba6ulx
 . ./setup-environment build_ut mainline