[Armbian build PR] - WIP: Allow reusing existing kernel tree as-is

This skips all git checkout, patching, configuring and cleaning when USE_EXISTING_KERNEL_TREE=yes is passed.

This can be useful when doing kernel development by making changes directly to the kernel sourcetree in cache/sources, while still allowing to use the compile.sh to use the same compilation commands, deb package building, ccache, etc. It is also useful when bisecting a kernel issue.

Can be used something like this:

USE_EXISTING_KERNEL_TREE=yes ./compile.sh default kernel

To produce a kernel deb in a few minutes ready to be installed on a device.

This is mostly a proof of concept to explore the usecase. In particular:

• I am not happy with the USE_EXISTING_KERNEL_TREE name yet.
• I wonder if this variable should maybe be split into different ones (e.g. for skipping git checkout, patching, config, clean already has one that can be used).
• Or maybe it would be better to split off some code out of compile_kernel() and expose a kernel_build and/or kernel_package command from ./compile.sh instead?

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