pw_log: Logging module first steps

This starts a logging module facade, with a frontend and a dumb IO based
backend. The initial version is lacking many features, like an API to
control enabling or disabling logs.

Change-Id: I3075aed33d9600a1190ca5237b5f7cfcfdc8f2ed
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
index 071aec1..cd9e2fe 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -103,6 +103,9 @@
 # Backend for the pw_dumb_io module.
 dir_pw_dumb_io_backend = ""
 
+# Backend for the pw_log module.
+dir_pw_log_backend = ""
+
 ############################## MODULE CONFIGS ##################################
 
 # Module configuration options for pw_boot_armv7m.