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=0Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/Kconfig

@@ -1548,8 +1548,20 @@ config ATAGS
 	  This is the traditional way of passing data to the kernel at boot
 	  time. If you are solely relying on the flattened device tree (or
 	  the ARM_ATAG_DTB_COMPAT option) then you may unselect this option
-	  to remove ATAGS support from your kernel binary.  If unsure,
-	  leave this to y.
+	  to remove ATAGS support from your kernel binary.
+
+config UNUSED_BOARD_FILES
+	bool "Board support for machines without known users"
+	depends on ATAGS
+	help
+	  Most ATAGS based board files are completely unused and are
+	  scheduled for removal in early 2023, and left out of kernels
+	  by default now.  If you are using a board file that is marked
+	  as unused, turn on this option to build support into the kernel.
+
+	  To keep support for your individual board from being removed,
+	  send a reply to the email discussion at
+	  https://lore.kernel.org/all/CAK8P3a0Z9vGEQbVRBo84bSyPFM-LF+hs5w8ZA51g2Z+NsdtDQA@mail.gmail.com/
 
 config DEPRECATED_PARAM_STRUCT
 	bool "Provide old way to pass kernel parameters"