Mbed TLS importer: don't import unnecessary files

The Mbed TLS importer accidentally imports Makefile and .gitignore
from Mbed TLS's `library/` directory. This commit restricts the
pattern to .h and .c files only, and removes the unnecessary files.

Author: LDong-Arm

connectivity/mbedtls/source/.gitignore

@@ -55,7 +55,7 @@ rsync:
 	#
 	# Copying mbed TLS into mbed library...
 	rm -rf $(TARGET_SRC)
-	rsync -a --exclude='*.txt' $(MBED_TLS_DIR)/library/ $(TARGET_SRC)
+	rsync -a --include='*.[hc]' --exclude='*' $(MBED_TLS_DIR)/library/ $(TARGET_SRC)
 	#
 	# Copying mbed TLS headers to mbed includes...
 	rm -rf $(TARGET_INC)