[build] Enable strict shift overflow warnings

Left shifts into the sign bit are often reported as potential
undefined behaviour by automated tools, which distracts from real
issues.

Now that all offending constant left shifts have been eliminated from
the codebase, enable -Wshift-overflow=2 to ensure that such shifts
cannot be reintroduced in future.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

diff --git a/src/Makefile.housekeeping b/src/Makefile.housekeeping
index ed21008..ddd4c8a 100644
--- a/src/Makefile.housekeeping
+++ b/src/Makefile.housekeeping
@@ -488,9 +488,9 @@
 ifeq ($(CCTYPE),gcc)
 CFLAGS		+= -ffreestanding
 CFLAGS		+= -fno-common
-CFLAGS		+= -Wall -W -Wformat-nonliteral
+CFLAGS		+= -Wall -W -Wformat-nonliteral -Wshift-overflow=2
 CFLAGS		+= -Wno-array-bounds -Wno-dangling-pointer
-HOST_CFLAGS	+= -Wall -W -Wformat-nonliteral
+HOST_CFLAGS	+= -Wall -W -Wformat-nonliteral -Wshift-overflow=2
 HOST_CFLAGS	+= -Wno-array-bounds -Wno-dangling-pointer
 endif
 CFLAGS		+= $(WORKAROUND_CFLAGS) $(EXTRA_CFLAGS)