Add patch to properly boot from mdadm via findiso boot option [Closes: issue1270]

For example grml-rescueboot uses findiso for booting the ISO.
When booting from a software RAID using mdadm then
/scripts/local-top/mdadm being used inside
/scripts/boot/9990-misc-helpers.sh leaks its output to the
environment, causing invalid data used inside the
mount_images_in_directory function. Because the invalid data
results in a failing mount_images_in_directory execution we end
up with failed boot and error message:

  "No supported filesystem images found at ...."

So instead redirect output of /scripts/local-top/mdadm to
/boot.log. Also make sure to check for existence of
/conf/conf.d/md before accessing it (the file doesn't always
exist).

While at it also make sure the same logic is used for
/scripts/local-top/lvm2.

Thanks: thinkgear <thinkgear@thinkgear.net> for reporting and providing a test VM