This allows to link with a modified version of btrfs-tools, that makes
availale mkfs.btrfs as library and SWupdate can call it :

https://github.com/kdave/btrfs-progs/compare/master...sbabic:btrfs-progs:swupdate



Standard default parameters are taken, there is currently no support for btrfs
tuning.

Signed-off-by: Stefano Babic <sbabic@denx.de>