buildzip.pl: more fixes

- cygwin's abs_path() can't operate on non existing path, so only
  evaluate abs_path("$install/something") if $install is set. (apply
  r27137 again)
- don't set $install to 0 since it's meant to be a string
- create the install prefix with mkdir -p before installing

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27341 a1c6a512-1295-4272-9138-f99709370657
1 file changed