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>