Add gdate dep to support SOURCE_DATE_EPOCH

[siva]

Summary

When SOURCE_DATE_EPOCH is set, u-boot's Makefile searches for a GNU-compatible date command.

Tested together with patch from https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291905

[siva]

LGTM, can confirm this fixes the issue. Waiting on maintainer approval now.

[cen.is.imba_gmail.com]

@manu could you check this out?

[siva]

In case of maintainer timeout, I will go ahead and commit this in a day or two.

[siva]

@manu Maintainer timeout. Let me know ASAP if you need any changes to v2 of this after I post it. I will be committing after testing all u-boot ports with SOURCE_DATE_EPOCH and once the ports tree is unfrozen.

@cen.is.imba_gmail.com I'm going to commandeer this and list you as the author to give you credit. I will update the diff with some necessary adjustments, please let me know if you need any fixes after that.

[siva]

Add BINARY_ALIAS for gdate and improve formatting.