Reread partition table after installing MBR to ensure devices exist

We need to execute blockdev after installing the MBR, otherwise
the devices might not exist yet as expected and executing syslinux
can fail therefore.

Closes: #33
Thanks: Darshaka Pathirana for debugging

grml2usb

@@ -875,6 +875,14 @@ def install_mbr(mbrtemplate, device, partition, ismirbsdmbr=True):
     proc = subprocess.Popen(["sync"])
     proc.wait()
 
+    logging.debug("Probing device via 'blockdev --rereadpt %s'", device)
+    proc = subprocess.Popen(["blockdev", "--rereadpt", device])
+    proc.wait()
+    if proc.returncode != 0:
+        raise Exception(
+            "Couldn't execute blockdev on '%s' (install util-linux?)", device
+        )
+
     set_rw(device)