XEN config can be enabled by other platforms (even it doesn't need to make
sense) that's why fix dependencies. XEN (xenbus.c) requires sscanf (also
pvblock needs it). And PVBLOCK is inside drivers/xen folder which requires
XEN to be enabled.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

Selecting this option can be handled in the Kconfig option itself, as it
is with BLK. Update this an drop the various 'select' clauses.

Signed-off-by: Simon Glass <sjg@chromium.org>

This option is fact really related to SPL. For U-Boot proper we always use
driver model for block devices, so CONFIG_BLK is enabled if block devices
are in use.

It is only for SPL that we have two cases:

- SPL_BLK is enabled, in which case we use driver model and blk-uclass.c
- SPL_BLK is not enabled, in which case (if we need block devices) we must
  use blk_legacy.c

Rename the symbol to SPL_LEGACY_BLOCK to make this clear. This is
different enough from BLK and SPL_BLK that there should be no confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>

Add initial infrastructure for Xen para-virtualized block device.
This includes compile-time configuration and the skeleton for
the future driver implementation.
Add new class UCLASS_PVBLOCK which is going to be a parent for
virtual block devices.
Add new interface type IF_TYPE_PVBLOCK.

Implement basic driver setup by reading XenStore configuration.

Signed-off-by: Andrii Anisov <andrii_anisov@epam.com>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>