SPDX is an international standard for documenting software license
requirements. Remove the existing headers and replace with a brief
SPDX preamble.
See: https://spdx.dev/use/specifications/
The script used to convert the files is added to "tools", and the
file header templates in headers/ are updated.
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
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