ARM: add CONFIG_UNUSED_BOARD_FILES

ARM: add CONFIG_UNUSED_BOARD_FILES

Based on the recent mailing list discussion, most board file support
has no remaining users and can be scheduled for removal early next
year.

If a board is still found to have users, it will remain for this round
but users are encouraged to migrate to devicetree based booting where
possible.

The timing is meant to ensure the next longterm supported kernel
still contains all the board files, giving another year of support
for potential users that did not speak up and would otherwise be
stuck on the v5.15.y longterm kernel from 2021.

Link: https://lore.kernel.org/all/CAK8P3a0Z9vGEQbVRBo84bSyPFM-LF+hs5w8ZA51g2Z+NsdtDQA@mail.gmail.com/
Link: https://docs.google.com/spreadsheets/d/1PL4dUUSieeXHzZhAn_Rnix32OTiCfN33sCQejpvI6ng/edit#gid=0
Signed-off-by: Arnd Bergmann <arnd@arndb.de>