log: Allow runtime selection of 'debug' log level

log: Allow runtime selection of 'debug' log level

Currently, we need to compile with -DDEBUG to implement debug-level
logging in the UIs and discover server.

Since we may not be able to easily replace a system's petitboot
binaries, this change introduces a -v|--verbose option to the discver
server and ncurses UI, which enables debug at runtime. We also move some
of the udev debug code out of an #ifdef DEBUG block.

Since petitboot is generally started on boot, we also add a little
infrastructure to pass -v to petitboot on certain system contitions:
either petitboot.debug on the kernel command line, or a petitboot,debug?
NVRAM property containing the value 'true'.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>