kernel: Clean up build command and rules

This cleans up the DTB handling logic so it's more obvious what's going
on and consolidates the final target into a single rule. Additionally
makes a function for kernel builds so that binary, modules, and headers
can be built separately using the same command line.

Change-Id: I2deda962b1605fcd120c4275bac2f19246c93342
1 file changed