board/broadcom/northstar: fix building custom images

Custom Northstar images allow installing (flashing) them using vendor UI
or CFE bootloader.

The change of DTS files paths ("broadcom/" prefix introduction in Linux
6.5+) prevented post script from generating them.

Adjust post-image.sh to deal with new paths.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/broadcom/northstar/post-image.sh

@@ -27,8 +27,9 @@ build_trx() {
 	rm -f $images/zImage.$device.lzma
 }
 
-devices="$(sed -n 's/^BR2_LINUX_KERNEL_INTREE_DTS_NAME="\([a-z0-9 \-]*\)"$/\1/p' ${BR2_CONFIG})"
+devices="$(sed -n 's/^BR2_LINUX_KERNEL_INTREE_DTS_NAME="\([^"]*\)"$/\1/p' ${BR2_CONFIG})"
 for device in $devices; do
+	device="${device#broadcom/}"
 	case "$device" in
 		"bcm4708-smartrg-sr400ac")
 			build_trx "$1" "$device"