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/am335x_evm.h b/include/configs/am335x_evm.h
index 339d4bd..9d80739 100644
--- a/include/configs/am335x_evm.h
+++ b/include/configs/am335x_evm.h
@@ -81,7 +81,7 @@
 		"bootm ${loadaddr}\0" \
 
 #define CONFIG_BOOTCOMMAND \
-	"if mmc rescan ${mmcdev}; then " \
+	"mmc dev ${mmcdev}; if mmc rescan; then " \
 		"echo SD/MMC found on device ${mmcdev};" \
 		"if run loadbootenv; then " \
 			"echo Loaded environment from ${bootenv};" \