Automatically get the kernel path

[nrj5k]

Kernel headers usually exist in either:

• /usr/src/
• /usr/src/kernels/

Can that be prefilled in the config directly? This should reduce the involvement of datacrumbs setup.

[nrj5k]

For a path /usr/src/linux-headers-$(uname) it loses the linux-headers part
Additionally, DATACRUMBS_KERNEL_PATH should be DATACRUMBS_KERNEL_VERSION in the cmake.