ostree: Mount boot partition from user space

In the current approach we assumed that boot/

- is on a separate boot partition
- boot partition is right before rootfs partition
- rootfs parition is provided as a *device node*
- has "ext2" filesystem (mount shell cmd tries to
  auto detect FS type, mount system call does not.)

The new approch does not assume any of the above, and
moves this logic from early user space (initramfs) to
user space. Initramfs by design should be used only to
prepare the rootfs, other tasks should be done later.

Change-Id: Idc429c8061c1fd486658e4b4d170ee487682cc91
Reviewed-by: Samuli Piippo <samuli.piippo@theqtcompany.com>