bcm27xx: backup /boot/cmdline.txt and restore early

If you want to use the Raspberry Pi UART, "console=serial0,115200" needs
to be removed from the kernel cmdline. This is done by editing
/boot/cmdline.txt. However, this file is not currently backed up during
sysupgrade, so this effectively breaks HATs that require the use of the
UART every sysupgrade.

Backup this file during sysupgrade, and restore it before rebooting.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

target/linux/bcm27xx/base-files/lib/upgrade/keep.d/platform

@@ -1 +1,2 @@
+/boot/cmdline.txt
 /boot/config.txt

target/linux/bcm27xx/base-files/lib/upgrade/platform.sh

@@ -92,7 +92,7 @@ platform_copy_config() {
 		mkdir -p /boot
 		[ -f /boot/kernel.img ] || mount -t vfat -o rw,noatime "/dev/$partdev" /boot
 		cp -af "$UPGRADE_BACKUP" "/boot/$BACKUP_FILE"
-		tar -C / -zxvf "$UPGRADE_BACKUP" boot/config.txt
+		tar -C / -zxvf "$UPGRADE_BACKUP" boot/cmdline.txt boot/config.txt
 		sync
 		unmount /boot
 	fi