make TR_EXCLUDE work for SUBDIRS

make TR_EXCLUDE work for SUBDIRS

this is needed for excluding subprojects we have no direct control over,
e.g., git submodules.

note that unlike TR_EXCLUDE for files, this matches SUBDIRS entries, not
actual project file names. wildcards also work, though.

Change-Id: I1d0a0410a28af4a60f045ee5bbac34540c14f0bd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>