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>

Makefile.inc

@@ -385,11 +385,13 @@ CFLAGS_common += -Wstrict-aliasing -Wshadow -Wdate-time
 CFLAGS_common += -fno-common -ffreestanding -fno-builtin -fomit-frame-pointer
 CFLAGS_common += -ffunction-sections -fdata-sections -fno-pie
 ifeq ($(CONFIG_COMPILER_GCC),y)
-CFLAGS_common += -Wno-packed-not-aligned
 CFLAGS_common += -fno-delete-null-pointer-checks
 # Don't add these GCC specific flags when running scan-build
 ifeq ($(CCC_ANALYZER_OUTPUT_FORMAT),)
+CFLAGS_common += -Wno-packed-not-aligned
 CFLAGS_common += -fconserve-stack
+# cf. commit f69a99db (coreboot: x86: enable gc-sections)
+CFLAGS_common += -Wno-unused-but-set-variable
 endif
 endif
 
@@ -439,11 +441,6 @@ ADAFLAGS_common += -gnatyN
 
 LDFLAGS_common := --gc-sections -nostdlib -nostartfiles -static --emit-relocs
 
-ifeq ($(CONFIG_COMPILER_GCC),y)
-# cf. commit f69a99db (coreboot: x86: enable gc-sections)
-CFLAGS_common += -Wno-unused-but-set-variable
-endif
-
 ifeq ($(CONFIG_WARNINGS_ARE_ERRORS),y)
 CFLAGS_common += -Werror
 endif