Consider adding LVM snapshot support

[rinigus]

To add snapshot support, we need to enable few kernel options:

Device Drivers --->
  [*] Multiple devices driver support (RAID and LVM) ---> [CONFIG_MD]
    <*/M>   Device mapper support                         [CONFIG_BLK_DEV_DM]
    <*/M/ >   Crypt target support                        [CONFIG_DM_CRYPT]
    <*/M/ >   Snapshot target                             [CONFIG_DM_SNAPSHOT]
    <*/M/ >   Thin provisioning target                    [CONFIG_DM_THIN_PROVISIONING]
    <*/M/ >   Mirror target                               [CONFIG_DM_MIRROR]

Reference: http://www.linuxfromscratch.org/blfs/view/svn/postlfs/lvm2.html

Request: https://together.jolla.com/question/205304/request-add-modules-required-for-lvm-snapshots-to-xperia-x-kernel/

However, to use it in practice, we need to be able to unmount /rootfs in rescue mode.