spl: fix calling "spl export .." more than once

running "spl export ..." more than once fails with:

Trying to execute a command out of order
Trying to execute a command out of order
Trying to execute a command out of order
Trying to execute a command out of order
Trying to execute a command out of order
Trying to execute a command out of order
ERROR prep subcommand failed!
Subcommand failed

reason is commmit:
35fc84fa: Refactor the bootm command to reduce code duplication

It used "state != BOOTM_STATE_START" but state is a bitfield, so
check if the bit BOOTM_STATE_START is not set. With this fix,
"spl export ..." can called more than once ...

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>