Add `ENABLE_ASSERTIONS` build option

Add the new build option `ENABLE_ASSERTIONS` that controls whether or
not assert functions are compiled out. It defaults to 1 for debug builds
and to 0 for release builds.

Additionally, a following patch will be done to allow this build option
to hide auxiliary code used for the checks done in an `assert()`. This
code is is currently under the DEBUG build flag.

Assert messages are now only printed if LOG_LEVEL >= LOG_LEVEL_INFO,
which is the default for debug builds.

This patch also updates the User Guide.

Change-Id: I1401530b56bab25561bb0f274529f1d12c5263bc
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/lib/stdlib/assert.c b/lib/stdlib/assert.c
index 3486e50..3c0bd16 100644
--- a/lib/stdlib/assert.c
+++ b/lib/stdlib/assert.c
@@ -32,15 +32,18 @@
 #include <debug.h>
 #include <platform.h>
 
-/*
- * This is a basic implementation. This could be improved.
- */
-void __assert (const char *function, const char *file, unsigned int line,
+void __assert(const char *function, const char *file, unsigned int line,
 		const char *assertion)
 {
+#if LOG_LEVEL >= LOG_LEVEL_INFO
+	/*
+	 * Only print the output if LOG_LEVEL is higher or equal to
+	 * LOG_LEVEL_INFO, which is the default value for builds with DEBUG=1.
+	 */
 	tf_printf("ASSERT: %s <%d> : %s\n", function, line, assertion);
 
 	console_flush();
+#endif
 
 	plat_panic_handler();
 }