sandbox: restore ability to access host fs through standard commands

Commit 95fac6ab4589 "sandbox: Use os functions to read host device tree"
removed the ability for get_device_and_partition() to handle the "host"
device type, and redirect accesses to it to the host filesystem. This
broke some unit tests that use this feature. So, revert that change. The
code added back by this patch is slightly different to pacify checkpatch.

However, we're then left with "host" being both:
- A pseudo device that accesses the hosts real filesystem.
- An emulated block device, which accesses "sectors" inside a file stored
  on the host.

In order to resolve this discrepancy, rename the pseudo device from host
to hostfs, and adjust the unit-tests for this change.

The "help sb" output is modified to reflect this rename, and state where
the host and hostfs devices should be used.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
V2:
* Fix typo due to fixing checkpatch and not recompiling:-(
* Fix "help sb" output.
sandbox: terminate os_dirent_ls() result list

Each node in the linked-list that os_dirent_ls() returns has its next
pointer set only when the next node is created. For the last node in the
list, there is no next node, so this never happens, and the next pointer
is never initialized. Explicitly initialize the next pointer so that it
isn't dangling. Without this, "sb ls" might crash.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
ARM: tegra: enable DFU too

Enable DFU protocol support (via the "dfu" command) on Tegra boards where
USB device/gadget mode is enabled.

This relies on a number of fixes to the ci_udc HW driver and DFU protocol
code.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
TEMP HACK: enable ext4 write for DFU testing
fs: fat: implement fat_size()

Add function fat_size() to allow code to query the size of a file before
reading it. This could be used to allocate an appropriately sized buffer,
or to set up a USB or network communication protocol to read the data in
chunks.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
fs: ext4: implement ext4fs_size()

Add function ext4fs_size() to allow code to query the size of a file
before reading it. This could be used to allocate an appropriately sized
buffer, or to set up a USB or network communication protocol to read the
data in chunks.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
fs: implement size/fatsize/ext4size

FIXME: Needs squashing etc.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
dfu: fix some issues with reads/uploads

FIXME: reword after updates to use "*size" command for fs...

DFU read support appears to rely upon dfu->read_medium() updating the
passed-by-reference len parameter to indicate the remaining size
available for reading.

dfu_read_medium_mmc() never does this, and the implementation of
dfu_read_medium_nand() will only work if called just once; it hard-codes
the value to the total size of the NAND device irrespective of read
offset.

I believe that overloading dfu->read_medium() is confusing. As such,
this patch introduces a new function dfu->get_medium_size() which can
be used to explicitly find out the medium size, and nothing else.
dfu_read() is modified to use this function to set the initial value for
dfu->r_left, rather than attempting to use the side-effects of
dfu->read_medium() for this purpose.

Due to this change, dfu_read() must initially set dfu->b_left to 0, since
no data has been read.

dfu_read_buffer_fill() must also be modified not to adjust dfu->r_left
when simply copying data from dfu->i_buf_start to the upload request
buffer. r_left represents the amount of data left to be read from HW.
That value is not affected by the memcpy(), but only by calls to
dfu->read_medium().

After this change, I can read from either a 4MB or 1.5MB chunk of a 4MB
eMMC boot partion with CONFIG_SYS_DFU_DATA_BUF_SIZE==1MB. Without this
change, attempting to do that would result in DFU read returning no data
at all due to r_left never being set.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
v2:
* Fix dfu_get_medium_size_mmc() to handle DFU_FS_FAT/EXT5 layouts too.
* Use "size" command to retrieve file size.