Use $ANDROID_SERIAL if no selector is specified

Like adb, read the ANDROID_SERIAL environment variable to select a
device by serial if no explicit selection (-s, -d, -e or --tcpip=<addr>)
is provided via the command line.

Fixes #3111 <#3111>

app/scrcpy.1

@@ -477,6 +477,10 @@ Push file to device (see \fB\-\-push\-target\fR)
 .B ADB
 Path to adb.
 
+.TP
+.B ANDROID_SERIAL
+Device serial to use if no selector (-s, -d, -e or --tcpip=<addr>) is specified.
+
 .TP
 .B SCRCPY_ICON_PATH
 Path to the program icon.

app/src/cli.c

@@ -660,6 +660,11 @@ static const struct sc_envvar envvars[] = {
         .name = "ADB",
         .text = "Path to adb executable",
     },
+    {
+        .name = "ANDROID_SERIAL",
+        .text = "Device serial to use if no selector (-s, -d, -e or "
+                "--tcpip=<addr>) is specified",
+    },
     {
         .name = "SCRCPY_ICON_PATH",
         .text = "Path to the program icon",
@@ -1651,6 +1656,14 @@ parse_args_with_getopt(struct scrcpy_cli_args *args, int argc, char *argv[],
         return false;
     }
 
+    if (selectors == 0) {
+        const char *env_serial = getenv("ANDROID_SERIAL");
+        if (env_serial) {
+            LOGI("Using ANDROID_SERIAL: %s", env_serial);
+            opts->serial = env_serial;
+        }
+    }
+
 #ifdef HAVE_V4L2
     if (!opts->display && !opts->record_filename && !opts->v4l2_device) {
         LOGE("-N/--no-display requires either screen recording (-r/--record)"