determine the platform number of disk partitions by looking
at MAXPARTITIONS/OLDMAXPARTITIONS in kernel sources, so that it wouldn't
need to be specified separately in MAKEDEV.conf

change platform MAKEDEV.conf to contain only MD targets and nothing else;
simplify the parsing in MAKEDEV.awk accordingly

MD part of unified MAKEDEV
this contains information about disk partitions used by platform,
and MD MAKEDEV targets, such as 'init', MD part of 'all',
as well as any other MD-specific targets not covered by MAKEDEV.tmpl