Fix syntax for displaying usage string in two scripts (Tails#6414).

Thanks to Kill Your TV for the initial patch.

config/chroot_local-includes/usr/local/bin/tails-boot-to-kexec

@@ -22,7 +22,7 @@ case "$1" in
       fi
       ;;
    *)
-      usage "$0 kernel|initrd"
+      echo "Usage: $0 kernel|initrd" >&2
       exit 3
 esac
 

config/chroot_local-includes/usr/local/bin/tails-get-bootinfo

@@ -24,7 +24,7 @@ case "$1" in
       echo "${LIVE_IMAGE_MOUNTPOINT}${initrd}"
       ;;
    *)
-      usage "$0 kernel|initrd"
+      echo "Usage: $0 kernel|initrd" >&2
       exit 3
 esac