mpy-cross: When reading from stdin, write output to stdout.

Unless -o is given, output defaults to stdout unless a source file is
given (in which case the source file name is used to derive an output
file name).

Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>

mpy-cross/main.c

@@ -90,7 +90,8 @@ STATIC int compile_and_save(const char *file, const char *output_file, const cha
         cm.context = m_new_obj(mp_module_context_t);
         mp_compile_to_raw_code(&parse_tree, source_name, false, &cm);
 
-        if (output_file != NULL && strcmp(output_file, "-") == 0) {
+        if ((output_file != NULL && strcmp(output_file, "-") == 0) ||
+            (output_file == NULL && strcmp(file, "-") == 0)) {
             mp_raw_code_save(&cm, (mp_print_t *)&mp_stdout_print);
         } else {
             vstr_t vstr;
@@ -121,7 +122,7 @@ STATIC int usage(char **argv) {
         "usage: %s [<opts>] [-X <implopt>] [--] <input filename>\n"
         "Options:\n"
         "--version : show version information\n"
-        "-o : output file for compiled bytecode (defaults to input with .mpy extension)\n"
+        "-o : output file for compiled bytecode (defaults to input filename with .mpy extension, or stdout if input is stdin)\n"
         "-s : source filename to embed in the compiled bytecode (defaults to input file)\n"
         "-v : verbose (trace various operations); can be multiple\n"
         "-O[N] : apply bytecode optimizations of level N\n"