Only use submodule if present

Enabling the USE_CRYPTO_SUBMODULE option causes problems if the
crypto submodule isn't present. For example, when building
mbed-crypto as a submodule, it should use error.c from the parent
project if USE_CRYPTO_SUBMODULE is set. However if the parent
project isn't present, then the build will fail. Only enable it
if the submodule actually exists.
diff --git a/scripts/abi_check.py b/scripts/abi_check.py
index f837f7a..30c3fe5 100755
--- a/scripts/abi_check.py
+++ b/scripts/abi_check.py
@@ -148,7 +148,8 @@
         my_environment = os.environ.copy()
         my_environment["CFLAGS"] = "-g -Og"
         my_environment["SHARED"] = "1"
-        my_environment["USE_CRYPTO_SUBMODULE"] = "1"
+        if os.path.exists(os.path.join(git_worktree_path, "crypto")):
+            my_environment["USE_CRYPTO_SUBMODULE"] = "1"
         make_output = subprocess.check_output(
             [self.make_command, "lib"],
             env=my_environment,