tools: kwbimage: Set BOOT_FROM by default to SPI

tools: kwbimage: Set BOOT_FROM by default to SPI

kwbimage must have valid blockid member instead of zero value. Thus if
config file does not contain BOOT_FROM command, use by default the value
for SPI booting (which is probably the most common).

Signed-off-by: Pali Rohár <[email protected]>
Signed-off-by: Marek Behún <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>