Set default toolchain parameters

[franck-exceenis]

Summary:

Set default toolchain parameters and allow to pass TOOLCHAIN_PREFIX in command line.

Changes:

• Most of the time cross.cmake will be used as toolcahin file.
• It's better to pass an argument than having to modify a file to make it work.

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[elsalahy]

@franck-exceenis Thanks for the pull request. I have a couple of concerns with the PR:

• I don't like the overhead of having the the compiler path at hand when running the cmake command
• You are relying on the cache, but in development most of the time you delete the cache (think of a folder restructure)
• Compiler default paths can be really long (see windows for example), sure we can use environment variables, but then you are back to where we started
• Using a default path will produce a confusing warning, and there is no such thing as a default path especially when working with multiple operating systems
• If you check the cmake help you will see -D <var>[:<type>]=<value> = Create or update a cmake cache entry. and for me this is perfect for changeable entries such as chosen app, build type

Please let me know what you think of these points.

[elsalahy]

Closed due to inactivity.