Documentation: kbuild: Remove obsolete dtc_cpp section

		clean-files += *.dtb

		DTC_FLAGS ?= -p 1024

    dtc_cpp

	This is just like dtc as describe above, except that the C pre-

	processor is invoked upon the .dtsp file before compiling the result

	with dtc.

	In order for build dependencies to work, all files compiled using

	dtc_cpp must use the C pre-processor's #include functionality and not

	dtc's /include/ functionality.

	Using the C pre-processor allows use of #define to create named

	constants. In turn, the #defines will typically appear in a header

	file, which may be shared with regular C code. Since the dtc language

	represents a data structure rather than code in C syntax, similar

	restrictions are placed on a header file included by a device tree

	file as for a header file included by an assembly language file.

	In particular, the C pre-processor is passed -x assembler-with-cpp,

	which sets macro __ASSEMBLY__. __DTS__ is also set. These allow header

	files to restrict their content to that compatible with device tree

	source.

	A central rule exists to create $(obj)/%.dtb from $(src)/%.dtsp;

	architecture Makefiles do no need to explicitly write out that rule.

--- 6.8 Custom kbuild commands

	When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand