mdadm: Fix config generation

The init script generated something like "DEVICE=/dev/sda" when it should
have been generating "DEVICE /dev/sda". mdadm errors on this. Patch by jow.

Also changed the default sendmail path to /usr/sbin/sendmail. No package
in LEDE provides /sbin/sendmail. msmtp provides /usr/sbin/sendmail so use
that.

Also add a patch to fix file paths for mdadm runtime files. mdadm currently
errors on them since /run is missing. Once /run is added to stock LEDE, this
patch can be removed.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[rewrap commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>