Revise stripping and splitting of debug information

We now support two styles of debug information generation, the '.debug' style,
which is the same as previously implemented.  This style simply splits the
debug information and makes it available in the same general directory.

  /bin/foo -> /bin/.debug/foo

The new 'debug-file-directory' style splits the debug information and places
it into the single debug-file-directory, /usr/lib/debug:

  /bin/foo -> /usr/lib/debug/bin/foo.debug

Both also find and copy all referenced source code to a new /usr/src/debug
directory.  This allows the -dbg files to be used for stand-a-lone debugging
on or off the target device.

File stripping is now handled as a seperate operation from file splitting.
This allows us to split the debug information, but also leave it in the
original file -- or prevent the debug information from being split.

Also enhance the comments within local.conf.sample to provide a better
understanding of the control the user has over debug file generation.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>