Vento:

Tested on 'master' branch (If I remember correctly this also happen on 'lede-17.01' and 'openwrt-18.06')

Execute 'make download V=s' and you'll see (sometimes) this ('/home/vento/buildop' is where I cloned the openwrt git):

make[2]: Entering directory '/home/vento/buildop/tools/mkimage'
mkdir -p /home/vento/buildop/dl
SHELL= flock /home/vento/buildop/tmp/.u-boot-2018.03.tar.bz2.flock -c ' /home/vento/buildop/scripts/download.pl "/home/vento/buildop/dl" "u-boot-2018.03.tar.bz2" "7e7477534409d5368eb1371ffde6820f0f79780a1a1f676161c48442cb303dfd" "" "http://mirror2.openwrt.org/sources" "ftp://ftp.denx.de/pub/u-boot" '

• curl -f --connect-timeout 20 --retry 5 --location --insecure http://mirror2.openwrt.org/sources/u-boot-2018.03.tar.bz2
  % Total % Received % Xferd Average Speed Time Time Time Current
  Dload Upload Total Spent Left Speed
  0 0 0 0 0 0 0 0 --:--:-- 0:00:13 --:--:-- 0
  curl: (22) The requested URL returned error: 404 Not Found
  Download failed.
• curl -f --connect-timeout 20 --retry 5 --location --insecure ftp://ftp.denx.de/pub/u-boot/u-boot-2018.03.tar.bz2
  % Total % Received % Xferd Average Speed Time Time Time Current
  Dload Upload Total Spent Left Speed
  0 0 0 0 0 0 0 0 --:--:-- 0:19:37 --:--:-- 0
  curl: (56) Recv failure: Connection timed out
  Download failed.
• curl -f --connect-timeout 20 --retry 5 --location --insecure https://sources.lede-project.org/u-boot-2018.03.tar.bz2
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 11.7M 100 11.7M 0 0 1150k 0 0:00:10 0:00:10 --:--:-- 1227k
make[2]: Leaving directory '/home/vento/buildop/tools/mkimage'
time: tools/mkimage/download#0.32#0.21#1201.76

As you can see 'make download' is trying to download the file 'u-boot-2018.03.tar.bz2' from these two links:

• 'http://mirror2.openwrt.org/sources/u-boot-2018.03.tar.bz2' which return 404. Browsing 'http://mirror2.openwrt.org/sources/' you can see that 'u-boot-date.tar.bz2' files "stops" at "2014.10".
• 'ftp://ftp.denx.de/pub/u-boot/u-boot-2018.03.tar.bz2' the first time it fails (waiting 20 minutes to reach timeout) the second time it works. Tryinng manually from a browser does not lead to any error and works as expected.