Make installation path of mksquashfs configurable (implements #612)

[TheAssassin]

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[TheAssassin]

@probonopd @azubieta review appreciated.

[probonopd]

While we are at it (stowing mksquashfs away in /usr/lib/appimagekit/), would it be a good time to do the same for runtime (to make cross-architecture AppImage generation easier)?

In this case, we should use architecture-dependent subdirectories of /usr/lib/appimagekit/ (like https://wiki.debian.org/Multiarch/LibraryPathOverview#Multilib_suffixes - before this is implemented we should agree on which architecture naming scheme we want to use, lib64, amd64, x86_64 - like always, the Linux world can't seem to agree...)

[TheAssassin]

We can do that, yes. But if we do want to do so, and also want to ship multiple runtimes for different architectures, I'd say we should build a separate package for the runtime. Then, we can build that package for all the different architectures, which can be installed separately on request.

This should go hand-in-hand with the plan to stop embedding runtimes in appimagetool, and moving on to using plain files.

[probonopd]

I'd say we should build a separate package for the runtime.

I know why I don't like to do distro packaging... ;-) but it "they" want to split it up, fine with me.

[azubieta]

Packagers can always make a patch in order to change the install paths, so what ever you chose is OK as long as you set the separated paths for each arch.