btrfs-progs: Fix init script

the init script calls the old btrfsctl util, which is not included in
btrfs-progs anymore. Update the init script to call "btrfs device scan"
which assembles multi device btrfs filesystems.

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>
btrfs-progs: fix linking to libattr

I got the following error:
mkfs.o: In function `add_xattr_item':
mkfs.c:(.text+0x5dc): undefined reference to `llistxattr'
mkfs.c:(.text+0x748): undefined reference to `getxattr'
props.o: In function `prop_compression':
props.c:(.text+0x154): undefined reference to `fsetxattr'
props.c:(.text+0x198): undefined reference to `fgetxattr'
props.c:(.text+0x240): undefined reference to `fgetxattr'
collect2: error: ld returned 1 exit status

Fix it, by forcing btrfs-progs to link to libattr. While at it,
correct the dependency to libattr (instead of the tool attr).

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>
btrfs-progs: shrink to commonly used progs

btrfs-progs contains several programs, which are rarely used. Shrink
the btrfs-progs package to contain only the most commonly used ones.
This removes:
- btrfs-convert:	convert an existing ext2-4 to btrfs
- btrfs-debug-tree:	debug tool
- btrfs-find-root:	unused in openwrt init scripts
- btrfs-image:		creates an compressed filesystem image
- btrfs-map-logical:	debug tool
- btrfs-show-super:	debug tool
- btrfstune:		change a few rarely used filesystem settings
- btrfs-zero-log:	debug tool
- fsck.btrfs:		only prints "BTRFS file system", use btrfsck

Following programs will be installed:
- btrfs:		utility to control all aspekts of btrfs
- mkfs.btrfs:		create btrfs-fs
- btrfsck:		symlink to btrfs, same as "btrfs check"

this allows to remove the libext2fs dependency and shrinks the
package from 1,5MB to 0,5MB.