mx51evk: correct list of possible BOOT_FROM values

Signed-off-by: Stefano Babic <sbabic@denx.de>

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