build: ignore empty files

Do not error out on empty files, just ignore them.

This can be easily reproduced by touch'ing an empty file in the
directory of an appliance created with --prepare.

src/mode_build.ml

@@ -46,6 +46,7 @@ and file_content =
 | Packages
 | Hostfiles
 | Excludefiles
+| Empty
 
 let rec string_of_file_type = function
   | GZip c -> sprintf "gzip %s" (string_of_file_content c)
@@ -56,6 +57,7 @@ and string_of_file_content = function
   | Packages -> "packages"
   | Hostfiles -> "hostfiles"
   | Excludefiles -> "excludefiles"
+  | Empty -> "empty"
 
 let rec build debug
     (copy_kernel, format, host_cpu,
@@ -250,6 +252,8 @@ and read_appliance debug basedir appliance = function
     (* Depending on the file type, read or unpack the file. *)
     let appliance =
       match file_type with
+      | Uncompressed Empty | GZip Empty | XZ Empty ->
+        appliance
       | Uncompressed ((Packages|Hostfiles|Excludefiles) as t) ->
         let chan = open_in file in
         let lines = input_all_lines chan in
@@ -294,7 +298,7 @@ and update_appliance appliance lines = function
         String.sub path 1 (n-1)
     ) lines in
     { appliance with excludefiles = appliance.excludefiles @ lines }
-  | Base_image -> assert false
+  | Base_image | Empty -> assert false
 
 (* Determine the [file_type] of [file], or exit with an error. *)
 and get_file_type file =
@@ -331,6 +335,7 @@ and get_file_content file buf len =
   else if len >= 2 && buf.[0] = '/' then Hostfiles
   else if len >= 2 && buf.[0] = '-' then Excludefiles
   else if len >= 1 && isalnum buf.[0] then Packages
+  else if len = 0 then Empty
   else error "%s: unknown file type in supermin directory" file
 
 and get_compressed_file_content zcat file =