fs/btrfs: add support for generating a btrfs image

fs/btrfs: add support for generating a btrfs image

This patch makes it possible to format the rootfs using btrfs. It
introduces the option; BR2_TARGET_ROOTFS_BTRFS.

When selected, the user is able to specify the filesystem size, label,
options, and node and sector sizes. The new files are based on
fs/ext2/{Config.in,ext2.mk}

Signed-off-by: Robert J. Heywood <robert.heywood@codethink.co.uk>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas:
 - fix issues pointed by Yann (duplicated empty line, missing quotes
   around default values for string options)
 - use -f option so that we don't have to remove the image file before
   creating it again
 - use the --byte-count option to set the filesystem size, which
   avoids the need for doing a "truncate -s"
 - remove the possible explanation of a mkfs.btrfs error. Indeed,
   mkfs.btrfs automatically extends the size of the image as needed,
   so the size passed can never be "too small".
 - fix check-package warnings in Config.in file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>