CONFIG_BTRFS=y instead of CONFIG_BTRFS=m

[dsx724]

Enable built-in support for BTRFS roots please. Checksumming, compression, and copy-on-write can greatly reduce the problems users encounter with MicroSD cards.

[ghost]

I would also love to use btrfs as root file system. Especially on small and/or slow SD cards the compression is very useful.

And with btrfs (with compress=zlib) I am able to put a gentoo stage3 + portage on my 4GB SD card. With ext4 I run out of space with the same tarballs on the same SD card.
In the end any compressing file system would help me (jffs2, …).

[grigorig]

BTRFS would massively bloat the kernel image and is still considered unstable. After my requests to include IPv6 in the kernel image were turned down, a feature that is roughly 30% of the size of BTRFS, I don't think this is going to happen and not particularly useful either.

[popcornmix]

Agreed.