Move log to library

Move the log routines to the petitboot library.  The log
routines are generic enough to be used for both server and
client.  Does not change the log source.

jk: move to lib/log/ instead of lib/

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>