config: catch and report unexpected arguments

after handling arguments, catch and report any remaining "non-option"
arguments.

for instance users could make the basic mistake of using
`lb config --bootloaders syslinux grub-efi`, i.e. failing to quote the
multiple bootloaders (i actually encountered a user doing this who swore
that "it just worked"). catching and reporting such mistakes could be
valuable to users.

previous behaviour:
```
$ lb config --bootloaders syslinux grub-efi
P: Updating config tree for a debian/buster/amd64 system
P: Symlinking hooks...
```

new behaviour:
```
$ lb config --bootloaders syslinux grub-efi
[2020-05-05 18:56:07] lb config --bootloaders syslinux grub-efi
E: Unexpected argument found: grub-efi
```

Gbp-Dch: Short

1 files changed, 9 insertions, 0 deletions

diff --git a/scripts/build/config b/scripts/build/config
index 2bd929bc1..da8f3ce2f 100755
--- a/scripts/build/config
+++ b/scripts/build/config
@@ -822,6 +822,15 @@ Local_arguments ()
 				;;
 		esac
 	done
+
+	# Any remaining arguments should be reported as an unexpected arguments error.
+	if [ $# -ne 0 ]; then
+		local ARG
+		for ARG in "${@}"; do
+			Echo_error "Unexpected argument found: %s" "${ARG}"
+		done
+		exit 1
+	fi
 }
 
 # Processing args such that we have a value for --config if given