zck_log: allow a callback for logging

zck_log: allow a callback for logging

When zck is used as library, the caller has already its own LOG system.
It is nice to redirect zck's log into the main logger. ZCK can already
write into a fd specified by the user, but this add complexity in case
it cannot be mapped, for example if LOG should be sent to network or is
managed in a different way. iNot only, information like loglevel are
lost.

Signed-off-by: Stefano Babic <sbabic@denx.de>