Don't tftp to unknown flash

If a board has a variable number of flash banks, there are empty entries
in flash_info[] and CFG_DIRECT_FLASH_TFTP is set, tftp boot fails with
"Outside available Flash". This patch skips flash banks with unknown
flash ids.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>

net/tftp.c

@@ -111,6 +111,8 @@ store_block (unsigned block, uchar * src, unsigned len)
 
 	for (i=0; i<CFG_MAX_FLASH_BANKS; i++) {
 		/* start address in flash? */
+		if (flash_info[i].flash_id == FLASH_UNKNOWN)
+			continue;
 		if (load_addr + offset >= flash_info[i].start[0]) {
 			rc = 1;
 			break;