boot/at91bootstrap3: license files hashes only valid for latest version

We can only know the details of the license files for known versions.  For
custom, older or newer versions, the license files may change, or may be
moved around.

So, do for at91bootstrap3 as was done for other packages in the recent
past, and only define the list of license files for the latest version.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/at91bootstrap3/at91bootstrap3.mk

@@ -20,7 +20,9 @@ AT91BOOTSTRAP3_SITE = $(call github,linux4sam,at91bootstrap,$(AT91BOOTSTRAP3_VER
 endif
 
 AT91BOOTSTRAP3_LICENSE = Atmel License
+ifeq ($(BR2_TARGET_AT91BOOTSTRAP3_LATEST_VERSION),y)
 AT91BOOTSTRAP3_LICENSE_FILES = main.c
+endif
 
 AT91BOOTSTRAP3_INSTALL_IMAGES = YES
 AT91BOOTSTRAP3_INSTALL_TARGET = NO