[build] Inhibit "skipping incompatible" message from ld

On 64-bit systems with both 32-bit and 64-bit libraries installed, ld
tends to generate noisy "skipping incompatible /usr/lib/libxxx.so"
messages when building elf2efi.c.

Fix by passing --no-warn-search-mismatch to ld.

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

src/Makefile.housekeeping

 #
 ELF2EFI_CFLAGS	:= -I$(BINUTILS_DIR)/include -I$(BFD_DIR)/include \
 		   -idirafter include -L$(BINUTILS_DIR)/lib -L$(BFD_DIR)/lib \
-		   -lbfd -liberty -lz
+		   -lbfd -liberty -lz -Wl,--no-warn-search-mismatch
 
 $(ELF2EFI32) : util/elf2efi.c $(MAKEDEPS)
 	$(QM)$(ECHO) "  [HOSTCC] $@"