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 <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>