configs: Fix usage of mmc rescan

Fix usage of 'mmc rescan' by many configs.  Proper use is
'mmc dev ${mmcdev}; mmc rescan' to set the mmc device and then rescan
the device.  'mmc rescan' itself does not take any arguments.

Signed-off-by: Andrew Bradford <andrew@bradfordembedded.com>
diff --git a/include/configs/mx51evk.h b/include/configs/mx51evk.h
index 34b0783..dcae537 100644
--- a/include/configs/mx51evk.h
+++ b/include/configs/mx51evk.h
@@ -182,7 +182,7 @@
 		"dhcp ${uimage}; bootm\0" \
 
 #define CONFIG_BOOTCOMMAND \
-	"if mmc rescan ${mmcdev}; then " \
+	"mmc dev ${mmcdev}; if mmc rescan; then " \
 		"if run loadbootscript; then " \
 			"run bootscript; " \
 		"else " \