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
MbedTLS.

This commit was generated using the following script:

# ========================
#!/bin/sh

# 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
$!N
/This file is part of Mbed TLS/Id
P
D
'
# ========================

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

scripts/config.py

@@ -7,7 +7,7 @@ Basic usage, to read the Mbed TLS or Mbed Crypto configuration:
     if 'MBEDTLS_RSA_C' in config: print('RSA is enabled')
 """
 
-## 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
@@ -21,8 +21,6 @@ Basic usage, to read the Mbed TLS or Mbed Crypto configuration:
 ## 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)
 
 import os
 import re