armv7: release: Stop installing BBB u-boot

BBB code was removed from GENERIC kernel as it's not functional.
Stop installing u-boot for a platform that we don't boot on.

Reviewed by: imp, gjb
Approved by: re@ (gjb)
MFC after: never
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39844