firmware: shift away from old-style firmware-xxx files

opnsense-update can read the upgrade hint itself.  We may have
to stash an ABI in there to reach to a different location without
the need to publish a symbolic link.

Move the firmware message to a data location for cleanliness.

plist

@@ -203,6 +203,7 @@
 /usr/local/opnsense/contrib/simplepie/phpunit.xml.dist
 /usr/local/opnsense/contrib/tzdata/iso3166.tab
 /usr/local/opnsense/contrib/tzdata/zone.tab
+/usr/local/opnsense/data/firmware/upgrade.html
 /usr/local/opnsense/data/proxy/template_error_pages/ERR_ACCESS_DENIED.html
 /usr/local/opnsense/data/proxy/template_error_pages/ERR_ACL_TIME_QUOTA_EXCEEDED.html
 /usr/local/opnsense/data/proxy/template_error_pages/ERR_AGENT_CONFIGURE.html
@@ -247,7 +248,6 @@
 /usr/local/opnsense/data/proxy/template_error_pages/ERR_ZERO_SIZE_OBJECT.html
 /usr/local/opnsense/data/proxy/template_error_pages/error-details.txt
 /usr/local/opnsense/data/proxy/template_error_pages/errorpage.css
-/usr/local/opnsense/firmware-message
 /usr/local/opnsense/mvc/app/cache/README
 /usr/local/opnsense/mvc/app/config/config.php
 /usr/local/opnsense/mvc/app/config/loader.php

src/opnsense/scripts/firmware/check.sh

@@ -329,8 +329,8 @@ fi
 
 packages_is_size="$(opnsense-update -SRp)"
 if [ -n "${packages_is_size}" ]; then
-    upgrade_major_message=$(cat /usr/local/opnsense/firmware-message 2> /dev/null | sed 's/"/\\&/g' | tr '\n' ' ')
-    upgrade_major_version=$(cat /usr/local/opnsense/firmware-upgrade 2> /dev/null)
+    upgrade_major_message=$(cat /usr/local/opnsense/data/firmware/upgrade.html | sed 's/"/\\&/g' | tr '\n' ' ')
+    upgrade_major_version=$(opnsense-update -vR)
     upgrade_needs_reboot="1"
     sets_upgraded="{\"name\":\"packages\",\"size\":\"${packages_is_size}\",\"current_version\":\"${product_version}\",\"new_version\":\"${upgrade_major_version}\",\"repository\":\"${product_repo}\"}"