usb: gadget: f_sdp: Provide filesize env variable for downloaded images

Currently it's not possible to get filesize of downloaded images and
it's impossible to automate some tasks in scripts. So this patch adds
`filesize` environment variable with size (as hex number in bytes) of
the last successfully downloaded file via `sdp` command.

Cc: Lukasz Majewski <lukma@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Reviewed-by: Stefan Agner <stefan.agner@toradex.com>
diff --git a/drivers/usb/gadget/f_sdp.c b/drivers/usb/gadget/f_sdp.c
index 00a9f88..ae97ab2 100644
--- a/drivers/usb/gadget/f_sdp.c
+++ b/drivers/usb/gadget/f_sdp.c
@@ -100,6 +100,7 @@
 	enum sdp_state			state;
 	enum sdp_state			next_state;
 	u32				dnl_address;
+	u32				dnl_bytes;
 	u32				dnl_bytes_remaining;
 	u32				jmp_address;
 	bool				always_send_status;
@@ -276,6 +277,7 @@
 		sdp->state = SDP_STATE_RX_FILE_DATA;
 		sdp->dnl_address = be32_to_cpu(cmd->addr);
 		sdp->dnl_bytes_remaining = be32_to_cpu(cmd->cnt);
+		sdp->dnl_bytes = sdp->dnl_bytes_remaining;
 		sdp->next_state = SDP_STATE_IDLE;
 
 		printf("Downloading file of size %d to 0x%08x... ",
@@ -355,6 +357,9 @@
 	if (sdp->dnl_bytes_remaining)
 		return;
 
+#ifndef CONFIG_SPL_BUILD
+	env_set_hex("filesize", sdp->dnl_bytes);
+#endif
 	printf("done\n");
 
 	switch (sdp->state) {