TMPDIR should not be a subdirectory of $(pwd)

[qdii]

https://github.com/OpenVPN/openvpn-build/blob/be29cd5c7575da438783bd11412c04f32dfde28c/windows-nsis/build-complete#L19

According to the UNIX specification:

[TMPDIR is] a pathname of a directory made available for programs that need a place to create temporary files.

I don't see why IMAGEROOT should be under $(pwd). $(pwd) has no guarantee to have enough space or even be writeable. It sounds to me that a subdirectory of TMPDIR should be used instead.