build: include -Wextra in generic builds

TF-A is more strict with compiler warnings in comparison to other
projects (notably Linux) for security and -Wextra enables a lot of
desirable warnings. This patch enables -Wextra by default (from W=1
previously) and reorganises the warning levels so that they can useful
when enabled and not just a build failure. This will help us move
towards fixing the warnings that are too many to fix at once and
enabling all W={1, 2} warnings.

The warning levels get new meanings:
* W=1: warnings we want the generic build to include but are too time
  consuming to fix at the moment. They re-enable warnings taken out for
  generic builds.
* W=2: warnings we want the generic build to include but cannot be
  enabled due to external libraries.
* W=3: warnings that are informative but not necessary and generally too
  verbose and frequently ignored.

Quality expectations for new contributions mean that generally they
should have no warnings up to W=2. To allow code to be developed with
them in mind, -Werror is disabled when W=x is set. This way enabling
warnings will not just fail the build due to technicalities we have and
contributors will be able to actually see if they get any.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ieb15ddd635d458a956a34b0f9d0ea2f81b9c0745

Makefile

@@ -350,27 +350,51 @@ ASFLAGS_aarch64		=	$(march64-directive)
 # General warnings
 WARNINGS		:=	-Wall -Wmissing-include-dirs -Wunused	\
 				-Wdisabled-optimization -Wvla -Wshadow	\
-				-Wno-unused-parameter -Wredundant-decls
+				-Wredundant-decls
+# stricter warnings
+WARNINGS		+=	-Wextra -Wno-trigraphs
+# too verbose for generic build
+WARNINGS		+=	-Wno-missing-field-initializers \
+				-Wno-type-limits -Wno-sign-compare \
+# on clang this flag gets reset if -Wextra is set after it. No difference on gcc
+WARNINGS		+=	-Wno-unused-parameter
 
 # Additional warnings
-# Level 1
+# Level 1 - infrequent warnings we should have none of
-WARNING1 := -Wextra
+# full -Wextra
-WARNING1 += -Wmissing-format-attribute
+WARNING1 += -Wsign-compare
-WARNING1 += -Wmissing-prototypes
+WARNING1 += -Wtype-limits
-WARNING1 += -Wold-style-definition
+WARNING1 += -Wmissing-field-initializers
 
-# Level 2
+# Level 2 - problematic warnings that we want
-WARNING2 := -Waggregate-return
+# zlib, compiler-rt, coreboot, and mbdedtls blow up with these
-WARNING2 += -Wcast-align
+# TODO: disable just for them and move into default build
-WARNING2 += -Wnested-externs
+WARNING2 += -Wold-style-definition
+WARNING2 += -Wmissing-prototypes
+WARNING2 += -Wmissing-format-attribute
+# TF-A aims to comply with this eventually. Effort too large at present
+WARNING2 += -Wundef
 
+# Level 3 - very pedantic, frequently ignored
 WARNING3 := -Wbad-function-cast
+WARNING3 += -Waggregate-return
+WARNING3 += -Wnested-externs
+WARNING3 += -Wcast-align
 WARNING3 += -Wcast-qual
 WARNING3 += -Wconversion
 WARNING3 += -Wpacked
 WARNING3 += -Wpointer-arith
 WARNING3 += -Wswitch-default
 
+# Setting W is quite verbose and most warnings will be pre-existing issues
+# outside of the contributor's control. Don't fail the build on them so warnings
+# can be seen and hopefully addressed
+ifdef W
+ifneq (${W},0)
+E	 ?= 0
+endif
+endif
+
 ifeq (${W},1)
 WARNINGS += $(WARNING1)
 else ifeq (${W},2)