bitbake.conf: Further cleanup compiler optimization flags

bitbake.conf: Further cleanup compiler optimization flags

Move the -pipe option out of the optimization flags and directly into
the flags variables since we always use it now.

Also move the debug prefix mapping there to match the nativesdk case
which already does this.

Fix the documentation and two recipe usages to match the change.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>