[build] Fix building under OpenBSD

Similarly to FreeBSD, OpenBSD requires the object format to be
specified as elf_i386_obsd rather than elf_i386.

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

src/arch/i386/Makefile

 ASFLAGS		+= --32
 ifeq ($(HOST_OS),FreeBSD)
 LDFLAGS		+= -m elf_i386_fbsd
+else ifeq ($(HOST_OS),OpenBSD)
+LDFLAGS		+= -m elf_i386_obsd
 else
 LDFLAGS		+= -m elf_i386
 endif