[tls] Add boringssl, mbedtls and picotls library

Add boringssl, mbedtls and picotls libraries as submodules into
third_party folder. Build scripts will be added later one by one.

Change-Id: I81cb332672cab898252f4261f20ce29f6f31889e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/37400
Reviewed-by: Varun Sharma <vars@google.com>
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
diff --git a/.gitmodules b/.gitmodules
index 317cd91..744de82 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -4,3 +4,15 @@
 [submodule "third_party/nanopb"]
 	path = third_party/nanopb
 	url = https://pigweed.googlesource.com/third_party/github/nanopb/nanopb
+[submodule "third_party/boringssl/src"]
+	path = third_party/boringssl/src
+	url = https://pigweed.googlesource.com/third_party/boringssl/boringssl
+	branch = master
+[submodule "third_party/mbedtls/src"]
+	path = third_party/mbedtls/src
+	url = https://pigweed.googlesource.com/third_party/github/ARMmbed/mbedtls
+	branch = master
+[submodule "third_party/picotls/src"]
+	path = third_party/picotls/src
+	url = https://github.com/h2o/picotls.git
+	branch = master