Move try_umount above cleanup()

Move try_umount above cleanup()

try_umount() replaced all umount calls in PR #264.

Unfortunately, the function is defined too late for a few of functions,
especially for cleanup(), which fails with:

   -> Failed (rc=1)
  /sbin/grml-debootstrap: line 260: try_umount: command not found
  Unexpected non-zero exit code 127 in /sbin/grml-debootstrap /sbin/grml-debootstrap /sbin/grml-debootstrap /sbin/grml-debootstrap /sbin/grml-debootstrap at line 260 329 1107 1179 0 detected!
  last bash command: try_umount 3 "${MNTPOINT}"/boot/efi
   -> Failed (rc=1)
  /sbin/grml-debootstrap: line 260: try_umount: command not found

Moved the try_umount above cleanup().

Closes: #271