uboot: improve help text of BR2_TARGET_UBOOT_BOARDNAME

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

boot/uboot/Config.in

@@ -9,6 +9,8 @@ config BR2_TARGET_UBOOT_BOARDNAME
 	help
 	  One of U-Boot supported boards to be built.
 	  This will be suffixed with _config to meet U-Boot standard naming.
+	  See boards.cfg in U-Boot source code for the list of available
+	  configurations.
 
 choice
 	prompt "U-Boot Version"