boot_serial: espressif: ESP32-S2 serial recovery mode interface

Add the serial adapter for ESP32-S2 for boot recovery and MCUMGR
communication.

Signed-off-by: Almir Okato <almir.okato@espressif.com>
diff --git a/docs/readme-espressif.md b/docs/readme-espressif.md
index 7588022..354854f 100644
--- a/docs/readme-espressif.md
+++ b/docs/readme-espressif.md
@@ -552,7 +552,7 @@
 ---
 ***Note***
 
-Supported on ESP32 and ESP32-C3.
+Supported on ESP32, ESP32-C3, ESP32-S2.
 
 ---