CMake: Use generated source files from parent

When building as a submodule of a parent project, like Mbed TLS, use the
parent projects generated source files (error.c, version.c,
version_features.c)

Author: Patater

library/CMakeLists.txt

@@ -29,7 +29,6 @@ set(src_crypto
     ecp_curves.c
     entropy.c
     entropy_poll.c
-    error.c
     gcm.c
     havege.c
     hkdf.c
@@ -65,10 +64,26 @@ set(src_crypto
     sha512.c
     threading.c
     timing.c
+    xtea.c
+)
+
+# For files generated by the parent project (Mbed TLS) when building Mbed
+# Crypto as a submodule, ensure that the parent project instance is used.
+if(USE_CRYPTO_SUBMODULE)
+set(src_crypto
+    ${src_crypto}
+    ${CMAKE_SOURCE_DIR}/library/version.c
+    ${CMAKE_SOURCE_DIR}/library/version_features.c
+    ${CMAKE_SOURCE_DIR}/library/error.c
+)
+else()
+set(src_crypto
+    ${src_crypto}
     version.c
     version_features.c
-    xtea.c
+    error.c
 )
+endif()
 
 set(src_x509
     certs.c