mx51evk: correct list of possible BOOT_FROM values

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/board/freescale/mx51evk/imximage.cfg b/board/freescale/mx51evk/imximage.cfg
index db09913..a875e8f 100644
--- a/board/freescale/mx51evk/imximage.cfg
+++ b/board/freescale/mx51evk/imximage.cfg
@@ -26,7 +26,7 @@
 # The syntax is taken as close as possible with the kwbimage
 
 # Boot Device : one of
-# spi_flash, nand, onenand, sd_card
+# spi, sd (the board has no nand neither onenand)
 
 BOOT_FROM	spi