pkg-stats: update set of files to skip

In BR sub-directory boot/ linux/ and package/ there are a few .mk files which
aren't <package>.mk files. These files shouldn't be taken into account
in package statistics.

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>