MT#64132 Update to grml2usb version v0.20.13

Ensure to follow upstream changes, as grml2usb had 139 commits between
v0.20.6 and v0.20.13[1], most importantly adding support for GRUB in
BIOS boot.

[1] See `git log --oneline --no-merges v0.20.6..v0.20.13`

Change-Id: Id37302710bda389e4482a65d22b8a49f83db7b39

build_iso.sh

@@ -16,7 +16,7 @@ TEMPLATES="templates"
 GRML_URL="${GRML_URL:-https://deb.sipwise.com/deployment-iso/grml/}"
 GRML_HASH_URL="${GRML_HASH_URL:-https://deb.sipwise.com/deployment-iso/grml/}"
 SIPWISE_ISO="sip_provider_${MR}_${DATE}.iso"
-GRML2USB_VERSION='v0.20.6'
+GRML2USB_VERSION='v0.20.13'
 
 usage () {
   echo "Usage: $0 compat <grml.iso> <mr version> <Debian dist>"