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>

tests/git-scripts/pre-commit.sh

@@ -2,7 +2,7 @@
 
 # pre-commit.sh
 #
-# Copyright (c) 2017, 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
@@ -16,8 +16,6 @@
 # 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)
 
 # Purpose
 #