board/boundarydevices: remove u-boot from genimage.cfg

Adding a comment to explain how to update the bootloader since it is
located in NOR flash memory.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/boundarydevices/common/genimage.cfg

@@ -1,13 +1,15 @@
+# Minimal SD card image for Boundary Devices platforms
+#
+# It does not need a boot section for a bootloader since it is booted
+# from its NOR flash memory.
+#
+# To update the bootloader, execute the following from U-Boot prompt:
+# => run upgradeu
+
 image sdcard.img {
 	hdimage {
 	}
 
-	partition u-boot {
-		in-partition-table = "no"
-		image = "u-boot.imx"
-		offset = 1024
-	}
-
 	partition rootfs {
 		partition-type = 0x83
 		image = "rootfs.ext4"