Fix "Directory nonexistent" error

[z3ntu]

I get the following error message, when I do make multirom_zip:

--- Creating out/target/product/FP2/multirom-20160422-v33-UNOFFICIAL-FP2.zip
system/extras/multirom/install_zip/rename_zip.sh: 28: system/extras/multirom/install_zip/rename_zip.sh: cannot create out/target/product/FP2/multirom-20160422-v33-UNOFFICIAL-FP2.zip.md5sum: Directory nonexistent
make: *** [out/target/product/FP2/multirom] Error 2

#### make failed to build some targets (01:05 (mm:ss)) ####

without this patch, with it everything works as expected.

[z3ntu]

@Tasssadar can you please merge this?