u-boot: add project's new download source

Das U-Boot project is transitioning their infrastructure [1],[2] and now publishes releases on their
GitLab [3]. Add it as the first download source and keep the existing sources which are currently
still available for historical releases.

Also update the U-Boot project web page [1] for the package URL and bump PKG_RELEASE for uboot-tools

[1]: https://lore.kernel.org/u-boot/20260511174116.GJ1858239@bill-the-cat/
[2]: https://lore.kernel.org/u-boot/20260727224922.GB1188@bill-the-cat/
[3]: https://git.u-boot-project.org/u-boot/u-boot/-/releases/

Signed-off-by: Ryan Leung <untilscour@protonmail.com>
Link: https://github.com/openwrt/openwrt/pull/25343
Signed-off-by: Robert Marko <robimarko@gmail.com>

tools/mkimage/Makefile

@@ -11,6 +11,7 @@ PKG_VERSION:=2026.07
 
 PKG_SOURCE:=u-boot-$(PKG_VERSION).tar.bz2
 PKG_SOURCE_URL:= \
+	https://git.u-boot-project.org/u-boot/u-boot/-/releases/v$(PKG_VERSION)/downloads \
 	https://mirror.cyberbits.eu/u-boot \
 	https://ftp.denx.de/pub/u-boot
 PKG_HASH:=78e8bfc382fe388f9b55aa1daf8c563522a037779b5d4c349d1415e381f1243e