build: add option to exclude executables/dirs from being stripped

Sometimes it may be desirable to keep debug symbols for some binaries and
libraries on the target. This commit introduces the config option
BR2_STRIP_EXCLUDE_FILES, which is interpreted as a list of such binaries
and libraries, and the option BR2_STRIP_EXCLUDE_DIRS, which indicates
directories excluded from stripping entirely.
These exclusions are passed to the find command in the target-finalize step.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
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>