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:
35fc84fa1f: 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>

common/cmd_bootm.c

@@ -78,7 +78,8 @@ static int do_bootm_subcommand(cmd_tbl_t *cmdtp, int flag, int argc,
 		return CMD_RET_USAGE;
 	}
 
-	if (state != BOOTM_STATE_START && images.state >= state) {
+	if (((state & BOOTM_STATE_START) != BOOTM_STATE_START) &&
+	    images.state >= state) {
 		printf("Trying to execute a command out of order\n");
 		return CMD_RET_USAGE;
 	}