brcm47xx: sysupgrade: fix device model detection

$(board_name) was providing content on "boardtype" (and optionally
"boardnum") NVRAM values. That function requires & expects more specific
and detailed model name extracted from the /proc/cpuinfo.

Fixes: f12a326 ("treewide: use the generic board_name function")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/brcm47xx/base-files/lib/upgrade/platform.sh

@@ -15,9 +15,9 @@ get_le_long_at() {
 }
 
 platform_expected_image() {
-	local machine=$(board_name)
+	local model="$(cat /tmp/sysinfo/model)"
 
-	case "$machine" in
+	case "$model" in
 		"Netgear WGR614 V8")	echo "chk U12H072T00_NETGEAR"; return;;
 		"Netgear WGR614 V9")	echo "chk U12H094T00_NETGEAR"; return;;
 		"Netgear WGR614 V10")	echo "chk U12H139T01_NETGEAR"; return;;