boot/uboot: add hash for license file

Fixes:
 >>> uboot 2019.07 Collecting legal info
 ERROR: No hash found for Licenses/gpl-2.0.txt

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/uboot/uboot.hash

@@ -1,2 +1,3 @@
 # Locally computed:
 sha256  bff4fa77e8da17521c030ca4c5b947a056c1b1be4d3e6ee8637020b8d50251d0  u-boot-2019.07.tar.bz2
+sha256  8177f97513213526df2cf6184d8ff986c675afb514d4e68a404010521b880643  Licenses/gpl-2.0.txt