The Windows Installer was ignoring any user-specified install location,
due to the wrong variable being set. Turns out we needed INSTALLFOLDER
and not TARGETDIR here. This simple rename allows users to install the
package anywhere.

This change also fixes problems for those who use Bash on Windows. The
right directory was already in the path, but there was no Bash script
available to execute. We now provide one, which performs the equivalent
of the batch file we already shipped. Now when people install, they'll
be able to use RBTools from Bash without having to do anything special.

Built a package and installed it to a custom location in my Desktop.

I was able to then run rbt from both a command prompt and bash shell.