dap_vendor: Buffer MSD_Write packets into 512-byte blocks.

[microbit-carlos]

Currently flashing a Universal Hex in "segments" or "blocks" format via WebUSB does not work.

This PR sends to the file streamer blocks of the same size as the VFS sector size, which is what vfs_manager does and some stream formats expect, like Universal Hex and in the future UF2.

This could have been implemented inside file_stream instead, but this way it mirrors what the MSC side is doing, buffering the 64 byte packets into 512-byte sectors before passing it to vfs_manager, which then sends it to file_stream.

One constrain is that DAPLink doesn't know when the last DAP.js ID_DAP_MSD_Write packet is sent, so we need to flush whatever is left in the buffer on ID_DAP_MSD_Close. So the "close" response could contain an error writing the last buffer.

There is also a memory hit, as the buffer takes 512 bytes of RAM. We could reuse the usb_buffer from vfs_manager.c (which is passed to usbd_msc.c, so that's where the 64-byte MSC packets are buffer into VFS sectors), not if you prefer to keep them separated. To be fair, using WebUSB flashing + MSD drag&drop at the same time will fail anyway, so probably wouldn't need to worry much about having two different components accessing the same buffer.

[microbit-carlos]

As described in the PR, this is the buffer that could be removed and usb_buffer from vfs_manager.c used instead.

DAPLink/source/daplink/drag-n-drop/vfs_manager.c

Line 129 in 82dc31d

static uint32_t usb_buffer[VFS_SECTOR_SIZE / sizeof(uint32_t)];

DAPLink/source/usb/usbd_msc.h

Line 35 in 82dc31d

extern U8 *USBD_MSC_BlockBuf;

DAPLink/source/daplink/drag-n-drop/vfs_manager.c

Lines 379 to 391 in 82dc31d

	static void build_filesystem()
	{
	// Update anything that could have changed file system state
	file_transfer_state = default_transfer_state;
	vfs_user_build_filesystem();
	vfs_set_file_change_callback(file_change_handler);
	// Set mass storage parameters
	USBD_MSC_MemorySize = vfs_get_total_size();
	USBD_MSC_BlockSize = VFS_SECTOR_SIZE;
	USBD_MSC_BlockGroup = 1;
	USBD_MSC_BlockCount = USBD_MSC_MemorySize / USBD_MSC_BlockSize;
	USBD_MSC_BlockBuf = (uint8_t *)usb_buffer;
	}

Left it as a TODO comment to come to an agreement before merging the PR.

[microbit-carlos]

When combining this PR + out of order PR + UF2, it fails building some ports due to insufficient .bss so I had to reuse the usb_buffer from vfs_manager.c in the end to save 512 bytes.

Force pushed, diff can be seen in:

• https://github.com/ARMmbed/DAPLink/compare/15d3016c1a51e8c97785f3048338a6abb225ad88..eba229a887eb327197ea70af1c6b12297fca3b96