user-guide.rst: Update n1sdp Building Kernel Modules Natively section

Following changes have been added:
1. Restructure the steps.
2. Added information and workaround regarding native kernel build
   failure.
3. Update kernel clone repository folder name.

Signed-off-by: sahil <sahil@arm.com>
Change-Id: Ic9a4cc5fa9c3fedadc49b42a516719f8be719d2a