Build system: Add fixes for scanbuild

- Exclude build flags that generate warnings when scanbuild is running
- Add the SCANBUILD_ARGS variable to abuild so we can pass in arguments
to scanbuild.
- Set the default scanbuild argument to -k (--keep-going) so that even
if an error occurs it continues with the scan.  This is similar to what
we do with coverity runs.

Change-Id: I82e7c13d7fd7432b43c17a31834ec82fca158a07
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/27595
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/abuild/abuild

@@ -469,7 +469,7 @@ function build_config
 		if [ "$scanbuild" = "true" ]; then
 			scanbuild_out=$TARGET/${BUILD_NAME}-scanbuild
 			rm -rf "${scanbuild_out}"
-			BUILDPREFIX="scan-build -o ${scanbuild_out}tmp"
+			BUILDPREFIX="scan-build ${SCANBUILD_ARGS} -o ${scanbuild_out}tmp"
 		fi
 		compile_target "${BUILD_NAME}"
 		if [ "$scanbuild" = "true" ]; then
@@ -667,6 +667,7 @@ while true ; do
 		--scan-build)   shift
 			scanbuild=true
 			customizing="${customizing}, scan-build"
+			SCANBUILD_ARGS=${SCANBUILD_ARGS:-'-k'}
 			;;
 		-y|--ccache)    shift
 			customizing="${customizing}, ccache"