boot/at91bootstrap3: add upstream URL in Config.in help text

Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 8084c33b39a64c6d2906296d042263a348fc0c99)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/at91bootstrap3/Config.in

@@ -8,6 +8,8 @@ config BR2_TARGET_AT91BOOTSTRAP3
 	  - Peripheral drivers such as PIO, PMC or SDRAMC...
 	  - Physical media algorithm such as DataFlash, NandFlash, NOR Flash...
 
+	  https://www.at91.com/linux4sam/bin/view/Linux4SAM/AT91Bootstrap
+
 if BR2_TARGET_AT91BOOTSTRAP3
 
 choice