configs: rock4se: use Bootlin toolchain

This moves the defconfig to the Bootlin glibc stable external toolchain
as per[1].

[1]: https://elinux.org/Buildroot:DeveloperDaysELCE2024#Rules_for_defconfigs

Signed-off-by: Patrik Olsson <johan.patrik.olsson@gmail.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

configs/rock4se_defconfig

@@ -1,6 +1,8 @@
 BR2_aarch64=y
 BR2_cortex_a72_a53=y
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_12=y
+BR2_TOOLCHAIN_EXTERNAL=y
+BR2_TOOLCHAIN_EXTERNAL_BOOTLIN=y
+BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_AARCH64_GLIBC_STABLE=y
 BR2_GLOBAL_PATCH_DIR="board/radxa/rock4se/patches"
 BR2_DOWNLOAD_FORCE_CHECK_HASHES=y
 BR2_TARGET_GENERIC_HOSTNAME="rock-4se"