know what to do with it - can I push it to you for inclusion in U-Boot (although we do not sell the naked board) or must I follow the provisions of the GPL (e.g. ship the source code with the device)?
You don't have to include the source with every shipped device. You could instead include a letter ("a written offer") explaining how and where they can get the source:
From u-boot/COPYING:
b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,