Add Espressif build status badge

Add new badge to easily check build status for Espressif CI.

Signed-off-by: Fabio Utzig <utzig@apache.org>

README.md

@@ -4,6 +4,7 @@
 [![Coverity Scan Build Status](https://scan.coverity.com/projects/12307/badge.svg)][coverity]
 [![Build Status (Sim)](https://github.com/mcu-tools/mcuboot/workflows/Sim/badge.svg)][sim]
 [![Build Status (Mynewt)](https://github.com/mcu-tools/mcuboot/workflows/Mynewt/badge.svg)][mynewt]
+[![Build Status (Espressif)](https://github.com/mcu-tools/mcuboot/workflows/Espressif/badge.svg)][espressif]
 [![Publishing Status (imgtool)](https://github.com/mcu-tools/mcuboot/workflows/imgtool/badge.svg)][imgtool]
 [![Build Status (Travis CI)](https://img.shields.io/travis/mcu-tools/mcuboot/main.svg?label=travis-ci)][travis]
 [![Apache 2.0](https://img.shields.io/badge/License-Apache%202.0-blue.svg)][license]
@@ -12,6 +13,7 @@
 [coverity]: https://scan.coverity.com/projects/mcuboot
 [sim]: https://github.com/mcu-tools/mcuboot/actions?query=workflow:Sim
 [mynewt]: https://github.com/mcu-tools/mcuboot/actions?query=workflow:Mynewt
+[espressif]: https://github.com/mcu-tools/mcuboot/actions?query=workflow:Espressif
 [imgtool]: https://github.com/mcu-tools/mcuboot/actions?query=workflow:imgtool
 [travis]: https://travis-ci.org/mcu-tools/mcuboot
 [license]: https://github.com/mcu-tools/mcuboot/blob/main/LICENSE