add download cache

overload 'wget' with a function,
so all wget downloads are cached in
ether 'sources' or ENV defined using $CACHEDIR.

directory 'sources' is already in .gitignore

checksum store is in root-dir of repo
and can be manually checked with:

( cd sources && sha256sum -c ../SHA256SUMS.sources )

introducing '$GITDIR' and enforce run in this dir.

rootfs.sh

@@ -4,6 +4,12 @@ set -ex
 QEMU_CMD="${1:-qemu-system-x86_64}"	# or 'chroot' or 'minikernel'
 QEMU_RAM="${2:-8G}"
 
+GITDIR="$PWD/$(dirname "$0")"
+if [ ! -f 'rootfs.sh' ]; then
+	echo 'must be run from base of repo'
+	exit 1
+fi
+
 pushd sysa
 
 # SYSTEM A
@@ -14,6 +20,35 @@ sudo mount -t tmpfs -o size=8G tmpfs tmp
 
 LOGFILE="$PWD/tmp/bootstrap.log"
 
+wget()
+{
+	local url="$1"
+	local dir="${CACHEDIR:-$GITDIR/sources}"
+	local file
+
+	file=$(basename "$url")
+	mkdir -p "$dir"
+
+	test -s "$dir/$file" || command wget -O "$dir/$file" "$url"
+	cp -v "$dir/$file" .
+	checksum_do "$dir" "$file"
+}
+
+checksum_do()
+{
+	local dir="$1"
+	local file="$2"
+	local line
+	local store="$GITDIR/SHA256SUMS.sources"
+
+	if line=$(grep "[[:space:]][[:space:]]$file"$ "$store"); then
+		(cd "$dir" && echo "$line" | sha256sum -c)
+	else
+		echo 'Checksum mismatch or not found!'
+		exit 1
+	fi
+}
+
 # base: mescc-tools-seed
 # copy in all the mescc-tools-seed stuff
 cp -r mescc-tools-seed/src/mescc-tools-seed/x86/* tmp