sysutils/u-boot-tools: update to 2025.10 and fix SOURCE_DATE_EPOCH build

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