Bump Linux to 6.12 LTS

It appears that the eMMC issue has been resolved so it's no longer
necessary to limit to 5.10. The official BeagleBone Debian images are on
6.12 as well.

The RT patches weren't available with this patchset. While RT support
was mainlined, it wasn't for 32-bit. If anyone actually uses RT support
with Nerves on BBB, it would be good to match their working
configuration. I don't think anyone actually does, though.

Author: fhunleth

README.md

@@ -16,7 +16,7 @@ and [PocketBeagle](https://beagleboard.org/pocket).
 | CPU                  | 1 GHz ARM Cortex-A8             |
 | Memory               | 512 MB DRAM                     |
 | Storage              | 4 GB eMMC Flash and MicroSD     |
-| Linux kernel         | 5.10 w/ BBB patches             |
+| Linux kernel         | 6.12 w/ BBB patches             |
 | IEx terminal         | UART `ttyS0`                    |
 | GPIO, I2C, SPI       | Yes - [Elixir Circuits](https://github.com/elixir-circuits) |
 | ADC                  | Yes                             |