pw_assert: Initial assert module

This starts the assert and assert_basic modules, which are the beginning
of our assert foundation. Much more is needed; in particular the "tests"
don't really do much other than check for compilation.

Screenshot
==========

   ▄████▄      ██▀███      ▄▄▄           ██████     ██░ ██
  ▒██▀ ▀█     ▓██ ▒ ██▒   ▒████▄       ▒██    ▒    ▓██░ ██▒
  ▒▓█ 💥 ▄    ▓██ ░▄█ ▒   ▒██  ▀█▄     ░ ▓██▄      ▒██▀▀██░
  ▒▓▓▄ ▄██▒   ▒██▀▀█▄     ░██▄▄▄▄██      ▒   ██▒   ░▓█ ░██
  ▒ ▓███▀ ░   ░██▓ ▒██▒    ▓█   ▓██▒   ▒██████▒▒   ░▓█▒░██▓
  ░ ░▒ ▒  ░   ░ ▒▓ ░▒▓░    ▒▒   ▓▒█░   ▒ ▒▓▒ ▒ ░    ▒ ░░▒░▒
    ░  ▒        ░▒ ░ ▒░     ▒   ▒▒ ░   ░ ░▒  ░ ░    ▒ ░▒░ ░
  ░             ░░   ░      ░   ▒      ░  ░  ░      ░  ░░ ░
  ░ ░            ░              ░  ░         ░      ░  ░  ░
  ░

  Welp, that didn't go as planned. It seems we crashed. Terribly sorry!

  CRASH MESSAGE

     Check failed: x (=50) > y (=51300): You SHOULD see this message

  CRASH FILE & LINE

     ../../pw_assert/assert_test.cc:46

  CRASH FUNCTION

     PigweedTestBody

==========

Change-Id: I9dbb460b3b33040d8009749f44b9fc174e2c8138
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
index 58d3037..b7c9ae4 100644
--- a/pw_vars_default.gni
+++ b/pw_vars_default.gni
@@ -94,6 +94,9 @@
 # All of these should default to empty strings. For target-specific defaults,
 # modify these variables in a target configuration file.
 
+# Backend for the pw_assert module.
+dir_pw_assert_backend = ""
+
 # Backend for the pw_boot module.
 dir_pw_boot_backend = ""