enable SUSE_BTRFS_SNAPSHOT_BOOTING only on i386-pc

enable SUSE_BTRFS_SNAPSHOT_BOOTING on i386-pc and not on other
architectures (s390, ppc) the ensure their config not affected. Also
make comments as suggested by jredinger.
Merge pull request #28 from WenhuaChang/master

enable SUSE_BTRFS_SNAPSHOT_BOOTING only on i386-pc