bcm53xx: exit if writing kernel during sysupgrade fails

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/bcm53xx/base-files/lib/upgrade/platform.sh

@@ -206,7 +206,7 @@ platform_pre_upgrade_trx() {
 	}
 
 	# Flash
-	mtd write /tmp/kernel.trx firmware
+	mtd write /tmp/kernel.trx firmware || exit 1
 	nand_do_upgrade /tmp/root.ubi
 }
 
@@ -252,7 +252,7 @@ platform_pre_upgrade_seama() {
 
 	# Flash
 	local kernel_size=$(sed -n 's/mtd[0-9]*: \([0-9a-f]*\).*"\(kernel\|linux\)".*/\1/p' /proc/mtd)
-	mtd write $dir/kernel.seama firmware
+	mtd write $dir/kernel.seama firmware || exit 1
 	mtd ${kernel_size:+-c 0x$kernel_size} fixseama firmware
 	nand_do_upgrade $dir/root.ubi
 }