Update copyright notices to use Linux Foundation guidance

As a result, the copyright of contributors other than Arm is now
acknowledged, and the years of publishing are no longer tracked in the
source files.

Also remove the now-redundant lines declaring that the files are part of

This commit was generated using the following script:

# ========================

# Find files
find '(' -path './.git' -o -path './3rdparty' ')' -prune -o -type f -print | xargs sed -bi '

# Replace copyright attribution line
s/Copyright.*Arm.*/Copyright The Mbed TLS Contributors/I

# Remove redundant declaration and the preceding line
/This file is part of Mbed TLS/Id
# ========================

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
diff --git a/configs/config-symmetric-only.h b/configs/config-symmetric-only.h
index 94e80ab..054cb9e 100644
--- a/configs/config-symmetric-only.h
+++ b/configs/config-symmetric-only.h
@@ -4,7 +4,7 @@
  * \brief Configuration without any asymmetric cryptography.
- *  Copyright (C) 2019, ARM Limited, All Rights Reserved
+ *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0
  *  Licensed under the Apache License, Version 2.0 (the "License"); you may
@@ -18,8 +18,6 @@
  *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  *  See the License for the specific language governing permissions and
  *  limitations under the License.
- *
- *  This file is part of mbed TLS (https://tls.mbed.org)