log: force DEBUG when LOG_DEBUG is activated

log: force DEBUG when LOG_DEBUG is activated

When CONFIG_LOG is activated, if LOG_DEBUG is defined in a file and
DEBUG is not defined the trace with debug() macro are not displayed,
because the parameter cond : _DEBUG = 0 is checked in debug_cond().

With this patch the define DEBUG, used to force the trace generated by
debug() macro, is linked with the define LOG_DEBUG, used to force the
trace generated by other macros (log_debug, dev_dbg, pr_debug).

We only need to define LOG_DEBUG in a file to activate all the
traces generated by any U-Boot debug macro, as it is described in
/doc/develop/logging.rst

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>