[build] Remove unnecessary dependency on zlib

The dependency on zlib seems to have been introduced in commit 3dd7ce1
("[efi] Allow building with non-system libbfd") as an indirect
requirement of either libbfd or libiberty when building on Mac OS X.
Since we no longer use either of these libraries, remove the
unnecessary link against zlib.

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

src/Makefile

 GENKEYMAP	:= ./util/genkeymap.pl
 DOXYGEN		:= doxygen
 LCAB		:= lcab
-ZLIB_DIR	:= /usr
 
 ###############################################################################
 #

src/Makefile.housekeeping

 #
 # The EFI image converter
 #
-ELF2EFI_CFLAGS	:= -I$(ZLIB_DIR)/include -idirafter include
-ELF2EFI_LDFLAGS	:= -L$(ZLIB_DIR)/lib -lz -Wl,--no-warn-search-mismatch
 
 $(ELF2EFI32) : util/elf2efi.c $(MAKEDEPS)
 	$(QM)$(ECHO) "  [HOSTCC] $@"
-	$(Q)$(HOST_CC) $(HOST_CFLAGS) $(ELF2EFI_CFLAGS) -DEFI_TARGET32 $< \
-		$(ELF2EFI_LDFLAGS) -o $@
+	$(Q)$(HOST_CC) $(HOST_CFLAGS) -idirafter include -DEFI_TARGET32 $< -o $@
 CLEANUP += $(ELF2EFI32)
 
 $(ELF2EFI64) : util/elf2efi.c $(MAKEDEPS)
 	$(QM)$(ECHO) "  [HOSTCC] $@"
-	$(Q)$(HOST_CC) $(HOST_CFLAGS) $(ELF2EFI_CFLAGS) -DEFI_TARGET64 $< \
-		$(ELF2EFI_LDFLAGS) -o $@
+	$(Q)$(HOST_CC) $(HOST_CFLAGS) -idirafter include -DEFI_TARGET64 $< -o $@
 CLEANUP += $(ELF2EFI64)
 
 $(EFIROM) : util/efirom.c $(MAKEDEPS)