config: kernel: Add support for configuring BTRFS to be built-in

config: kernel: Add support for configuring BTRFS to be built-in

Add the KERNEL_BTRFS_FS config option so that targets can select
whether BTRFS support must be built-in.

Select this option (alongside KERNEL_BTRFS_FS_POSIX_ACL) from the
layerscape/armv8_64b subtarget instead of enabling it in
target/linux/layerscape/armv8_64b/config-* files.

Move disabling of CONFIG_BTRFS_FS_CHECK_INTEGRITY into generic configs.

This makes it possible for OpenWRT to be built with built-in BTRFS
support on specific boards, instead of whole targets.

Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://github.com/openwrt/openwrt/pull/15990
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>