u-boot.mk: switch download to https

ftp can cause problems on some networks switch primary download location
to https and add another mirror

Signed-off-by: Lucian Cristian <lucian.cristian@gmail.com>

include/u-boot.mk

@@ -3,7 +3,8 @@ PKG_NAME ?= u-boot
 ifndef PKG_SOURCE_PROTO
 PKG_SOURCE = $(PKG_NAME)-$(PKG_VERSION).tar.bz2
 PKG_SOURCE_URL = \
-	https://sources.openwrt.org \
+	https://mirror.cyberbits.eu/u-boot \
+	https://ftp.denx.de/pub/u-boot \
 	ftp://ftp.denx.de/pub/u-boot
 endif