pkg-utils.mk: add utility functions find*clauses and notfirstword

This patch adds a few utility functions to pkg-utils.mk.

Functions finddirclauses and findfileclauses help in building a find command
that skips a set of directories and performs operations on a set of files.
This pattern can for example be used to keep certain files or directories from
being stripped, or to remove certain files from a package installation.

The notfirstword function is the inverse of the 'firstword' function in make:
it returns all but the first word.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>