Allow to override iso build date with SOURCE_DATE_EPOCH

[bmwiedemann]

Allow to override iso build date with SOURCE_DATE_EPOCH
in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This date call works with GNU date and FreeBSD/MacOSX date.

[mcb30]

Thanks. I think it probably makes sense to check for SOURCE_DATE_EPOCH and, if set, append the arguments to ISOARGS. The existing logic will then take care of choosing a mkisofs equivalent that is capable of support those arguments. In particular, this would mean that a system with both genisoimage and xorrisofs installed would choose xorrisofs when SOURCE_DATE_EPOCH was specified.

Please do include the ability to use BSD date as per https://reproducible-builds.org/docs/source-date-epoch/

Also: a UEFI .iso image will include an embedded FAT filesystem: does genfsimg need to also do something to set the relevant timestamps within this?

[bmwiedemann]

I updated the PR to incorporate the improvements. It makes ipxe.iso reproducible for me, so I would consider UEFI out-of-scope for now.

[mcb30]

Merged with modifications as 0d68d7151. For me, this still left the isohybrid MBR ID as non-deterministic: this is addressed in 74c54461c