boot/uboot: change from FTP to HTTPS for official releases

The HTTPS URL seems to be more reliable and quicker for download than
FTP.  FTP may also be a blocked protocol on some sites and in CI/CD
setups.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/uboot/uboot.mk

@@ -33,7 +33,7 @@ UBOOT_SITE = $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_REPO_URL))
 UBOOT_SITE_METHOD = svn
 else
 # Handle stable official U-Boot versions
-UBOOT_SITE = ftp://ftp.denx.de/pub/u-boot
+UBOOT_SITE = https://ftp.denx.de/pub/u-boot
 UBOOT_SOURCE = u-boot-$(UBOOT_VERSION).tar.bz2
 endif