Add support for MkImg for making BBC micro disk images

docs/api/bbcmicro-programming-guide.md

@@ -6,7 +6,9 @@ The default configuration file puts the start address for the program at $0E00.
 
 The compiler outputs two files: a raw machine code file without an extension and a `.inf` file with file metadata.
 To use the file, you need to put it on a disk or a disk image.
-You can for example use tools like BBC Disk Explorer.
+You can for example use tools like BBC Disk Explorer (GUI) or MkImg (command line):
+
+    MkImg disk_image.ssd output_directory
 
 After putting it on a disk, the file can be run with:
 

src/main/scala/millfork/Main.scala

@@ -161,7 +161,7 @@ object Main {
           val start = platform.codeAllocators(bankName).startAt
           val codeLength = code.length
           Files.write(Paths.get(prgOutput +".inf"),
-            s"${path.getFileName} ${start.toHexString} ${start.toHexString} ${codeLength.toHexString}".getBytes(StandardCharsets.UTF_8))
+            f"${path.getFileName}%s ${start}%04X ${start}%04X ${codeLength}%04X".getBytes(StandardCharsets.UTF_8))
         }
     }
     errorReporting.debug(s"Total time: ${Math.round((System.nanoTime() - startTime)/1e6)} ms")