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/Makefile b/Makefile
index 02aa50e..d03c2bf 100644
--- a/Makefile
+++ b/Makefile
@@ -52,8 +52,9 @@
 
 include ${MAKE_HELPERS_DIRECTORY}defaults.mk
 
-# ASM_ASSERTION enabled for DEBUG builds only
+# Assertions enabled for DEBUG builds by default
 ASM_ASSERTION			:= ${DEBUG}
+ENABLE_ASSERTIONS		:= ${DEBUG}
 ENABLE_PMF			:= ${ENABLE_RUNTIME_INSTRUMENTATION}
 PLAT				:= ${DEFAULT_PLAT}
 
@@ -446,6 +447,7 @@
 $(eval $(call assert_boolean,CTX_INCLUDE_FPREGS))
 $(eval $(call assert_boolean,DEBUG))
 $(eval $(call assert_boolean,DISABLE_PEDANTIC))
+$(eval $(call assert_boolean,ENABLE_ASSERTIONS))
 $(eval $(call assert_boolean,ENABLE_PLAT_COMPAT))
 $(eval $(call assert_boolean,ENABLE_PMF))
 $(eval $(call assert_boolean,ENABLE_PSCI_STAT))
@@ -482,6 +484,7 @@
 $(eval $(call add_define,COLD_BOOT_SINGLE_CPU))
 $(eval $(call add_define,CTX_INCLUDE_AARCH32_REGS))
 $(eval $(call add_define,CTX_INCLUDE_FPREGS))
+$(eval $(call add_define,ENABLE_ASSERTIONS))
 $(eval $(call add_define,ENABLE_PLAT_COMPAT))
 $(eval $(call add_define,ENABLE_PMF))
 $(eval $(call add_define,ENABLE_PSCI_STAT))