Simplify process_wait()

The function process_wait() returned a bool (true if the process
terminated successfully) and provided the exit code via an output
parameter exit_code.

But the returned value was always equivalent to exit_code == 0, so just
return the exit code instead.

app/src/util/process.c

@@ -8,8 +8,8 @@ process_check_success(process_t proc, const char *name) {
         LOGE("Could not execute \"%s\"", name);
         return false;
     }
-    exit_code_t exit_code;
-    if (!process_wait(proc, &exit_code)) {
+    exit_code_t exit_code = process_wait(proc);
+    if (exit_code) {
         if (exit_code != NO_EXIT_CODE) {
             LOGE("\"%s\" returned with value %" PRIexitcode, name, exit_code);
         } else {