Change fast-setup.sh to use shallow-since

Shallow cloning will not work since we are now referring to active
upstream URL.
Use git clone with --shallow-since to make it work and also reduce the
time for cloning submodules. This works for Git >= 2.11

fast-setup.sh

@@ -41,9 +41,17 @@ fi
 
 echo "Updating and cloning submodules, this may take a long time"
 git config submodule.riscv-gnu-toolchain.update none
-git config -f .gitmodules submodule.linux.shallow true
-git config -f .gitmodules submodule.qemu.shallow true
-git config -f .gitmodules submodule.buildroot.shallow true
+
+# shallow clone submodules ahead of time (Git must be > 2.11)
+if [ ! -d linux ]; then
+  git clone --shallow-since=2019-09-14 https://github.com/torvalds/linux.git linux
+fi
+if [ ! -d buildroot ]; then
+  git clone --shallow-since=2019-08-29 https://github.com/buildroot/buildroot.git buildroot
+fi
+if [ ! -d qemu ]; then
+  git clone --shallow-since=2018-08-14 https://github.com/qemu/qemu.git qemu
+fi
 
 git submodule sync --recursive
 git submodule update --init --recursive