bootlinux: add v6.15-rc2 and config-next-20250417 config

Few bells and whistles enabled:

- CONFIG_QFMT_V2
  As mentioned by Theodore Ts'o, this is needed for testing quota [0]
- CONFIG_DEBUG_INFO_BTF
  useful for the latest blkalgn
- CONFIG_GDB_SCRIPTS
  useful for running gdb

[0] https://lkml.kernel.org/r/20250417164907.GA6008@mit.edu

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

Author: mcgrof