[ipxe-devel] reproducible builds

[Neil Roza]

Hi ipxe-devel,

Please find the attached diff representing a patch I would like to submit
for your consideration. This is a small change to the
`src/Makefile.housekeeping` that makes the generation of most artifacts
(notably not `*.usb` images) deterministic.

The scariest change here is the removal of the `BUILD_ID_CMD` in favor of
an inlined shell snippet where the `_build_id` symbol is defined. In
keeping with the comments that specify a unique `_build_id` for each
`$(BIN)/%.tmp`, I use the first 8 characters of the md5sum of the target,
in the expected base-prefixed hexadecimal representation. Calculating the
likelihood of collisions I leave as an exercise to the reviewer. :D

The `BUILD_TIMESTAMP` assignment has been changed to allow environment
variable overriding, but it defaults to `SOURCE_DATE_EPOCH`. The source
date epoch can also be overridden; it defaults to the Unix timestamp of the
current git HEAD commit.

I like reproducible builds, but I recognize that others have different
concerns. I'm happy to change what needs changing.


-- 
Neil Roza
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