bcm27xx: fix unmounting /boot after sysupgrade

Due to a typo, /boot is not properly unmounted after copying the backup
file to it. Fix the typo to solve this.

Fixes: 246916ddf4 ("brcm2708: use x86's upgrade scripts for all rpi targets")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

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

@@ -94,6 +94,6 @@ platform_copy_config() {
 		cp -af "$UPGRADE_BACKUP" "/boot/$BACKUP_FILE"
 		tar -C / -zxvf "$UPGRADE_BACKUP" boot/cmdline.txt boot/config.txt
 		sync
-		unmount /boot
+		umount /boot
 	fi
 }