log: Add LOG_SET_PREFIX() macro

log: Add LOG_SET_PREFIX() macro

LOG_SET_PREFIX() sets a logging prefix for the current block. The
prefix is prepended to every logging message in the block, followed
by ": ". If a logging prefix is already configured, it is overridden
for the duration of the block, after which it is restored.

A use case for this macro is when we're operating on an image or
directory (using --root or --image). We can use LOG_SET_PREFIX() to
prefix all logging messages with the directory or image that we're
operating on.