ci: mynewt: Update to install upstream Mbed-TLS

Update after recent removal of Mbed-TLS from Mynewt tree; now using
upstream from Mbed-TLS github repo.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/ci/mynewt_install.sh b/ci/mynewt_install.sh
index b7b065e..3ea0534 100755
--- a/ci/mynewt_install.sh
+++ b/ci/mynewt_install.sh
@@ -38,6 +38,10 @@
     # nrfx is now taken from original repository
     git clone --depth=1 --branch v2.8.0 https://github.com/NordicSemiconductor/nrfx.git repos/nordic-nrfx
     [[ $? -ne 0 ]] && exit 1
+
+    # Mbed-TLS is now taken from original repository
+    git clone --depth=1 --branch v2.28.3 https://github.com/Mbed-TLS/mbedtls.git repos/mbedtls
+    [[ $? -ne 0 ]] && exit 1
 }
 
 arm_toolchain_install() {