Currently, the kernel doesn't have the btrfs driver compiled in.

(Probably next week, I'll have some time to try to compile it, as a stop-gag measure until Jolla adds it officially).

So you can't mount it for now.

You'll have currently to go for EXT4, F2FS,
or you can try UDF (which is included, meaning you can have a Log-structured/CoW, cross-platform(*) and non-patented filesystem).

If you urgently *need* to access files from a BTRFS partition (e.g.: the µSDCX card of a dead Jolla 1), you could install 'btrfs-progs' and use ' btrfs restore /dev/mmc<bla>' to extract files without mounting them (that doesn't use the kernel and should work without the driver).

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(*) : pay attention that currently, there's a small incompatibility between Microsoft Windows and Apple Mac OS X.
One can only use UDF on *raw* flashmedia, while the other can only use UDF on *partitionned* flashmedia.
(Linux doesn't give a donkey's arse and can use either).
So you *MUST* use the trick by JElchison I've linked to make a hack partition table that point to the whole raw flash media, thus both Windows and OS X will see what they need.

(And in the specific case of Sailfish, I think sd-mount.sh will mount the raw media first, and then refuse to mount the partition because it's already mounted... must test)