sandbox: block driver using host file/device as backing store

A simple "host" block driver using any host file/device
as backing store.

Adds two sb subcommands for managing host device bindings:
sb bind <dev> [<filename>] - bind "host" device to file
sb info [<dev>]            - show device binding & info

Signed-off-by: Henrik Nordstrom <henrik@henriknordstrom.net>

Series-to: uboot
Series-cc: Simon Glass <sjg@chromium.org>
Series-version: 3

Series-changes: 3
- blank line between declarations and code
- declarations at top
- rename sandboxblockdev.h to sandbox-blockdev.h
- correct ordering in drivers/block/Makefile
- errno return from host_dev_bind (was command return)
- puts instead of printf where applicable

Series-changes: 2
- add "sb info" command for querying current bound devices
- make filename optional to "sb bind". Unbinds if not given
- verify argument correctness in "sb bind"
- add HOST device type name for part list
- make "sb bind" immediately attach the device instead of on first use
- enable EFI/GUID partitions as in sandbox config

Todo:
- Documentation
- Testsuite