setup-bootable: use stat to calculate needed disk space

setup-bootable.in

@@ -136,11 +136,16 @@ fi
 rm -rf "$destdir"/.new "$destdir"/.old
 
 # check that we have the space we need
-needed_space=$(cd "$srcdir" && du -s -c boot apks syslinux.cfg .alpine-release | awk '$2 == "total" {print $1}')
-[ -n "$verbose" ] && echo "Needed space:    $needed_space kB"
-
-available_space=$(df -k "$destdir" | tail -n 1 | awk '{print $4}')
-[ -n "$verbose" ] && echo "Available space: $available_space kB"
+# we calculate on MB since shell arthimetic gets problems with big disks
+# and bytes.
+needed_space=$(cd "$srcdir" && du -m -s -c boot apks syslinux.cfg .alpine-release | awk '$2 == "total" {print $1}')
+[ -n "$verbose" ] && echo "Needed space:    $needed_space MiB"
+
+free_blocks=$(stat -f -c "%f" "$destdir")
+block_size=$(stat -f -c "%s" "$destdir")
+blocks_per_mb=$(( 1024 * 1024 / $block_size))
+available_space=$(( $free_blocks / $blocks_per_mb ))
+[ -n "$verbose" ] && echo "Available space: $available_space MiB"
 [ $available_space -lt $needed_space ] && die "Not enough space on $destdir. Aborting."
 
 # copy the files to .new