fs: implement size/fatsize/ext4size

These commands may be used to determine the size of a file without
actually reading the whole file content into memory. This may be used
to determine if the file will fit into the memory buffer that will
contain it. In particular, the DFU code will use it for this purpose
in the next commit.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
v2: New patch
dfu: fix some issues with reads/uploads

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,
  by calling the recently introduced "size" command.
dfu: add write error handling

Fix calls to dfu_write() and dfu_flush() to detect errors in the I/O
itself. This could happen due to problems with the storage medium, or
simply when trying to write a FAT/ext file that is larger than the buffer
dfu_mmc.c maintains for this purpose.

Signal the error by switching the DFU state/status. This will be picked
up by the DFU client when it sends the next DFU request. Note that errors
can't simply be returned from e.g. dnload_request_complete(), since that
function has no way to pass errors back to the DFU client; a call to
dnload_request_complete() simply means that a USB OUT completed.

This error state/status needs to be cleared when the next DFU client
connects. While there is a DFU_CLRSTATUS request, no DFU client seems to
send this. Hence, clear this when selecting the USB alternate setting on
the USB interface.

Finally, dfu.c relies on a call to dfu_flush() to clear up the internal
state of the write transaction. Now that errors in dfu_write() are
detected, dfu_flush() may no longer be called for every transaction.
Separate out the cleanup code into a new function, and call it whenever
dfu_write() fails, as well as from any call to dfu_flush().

Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
This probably depends on "dfu: fix some issues with reads/uploads",
for context if nothing else.