[build] Fix building on OpenBSD 5.4

OpenBSD 5.4 seems to generate dynamically linked binaries by default,
which breaks our build process.  Fix by forcing the linker to always
create static binaries.

Reported-by: Jiri B <jirib@devio.us>
Tested-by: Jiri B <jirib@devio.us>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 endif
 LDFLAGS		+= --gc-sections
 
+# Force creation of static binaries (required for OpenBSD, does no
+# harm on other platforms).
+#
+LDFLAGS		+= -static
+
 # compiler.h is needed for our linking and debugging system
 #
 CFLAGS		+= -include include/compiler.h