This fixes the reproducible build when SOURCE_DATE_EPOCH is specified.
* moved to upstream-default gnutls since it doesn't correctly build with the USES=ssl and openssl flags
* added BUILD_DEPENDS on gdate from coreutils to handle SOURCE_DATE_EPOCH correctly
* cleaned up and simplified make target a bit
* removed CONFLICTS since it is nonexistent
Tested by building all dependent sysutils/u-boot* ports.
PR: 292260
Approved by: maintainer timeout (1+ month), lwhsu (mentor, implicitly)
Sponsored by: The FreeBSD Foundation
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