kbuild: Avoid including architecture-specific Makefile twice

kbuild: Avoid including architecture-specific Makefile twice

Stranges errors are observed when building U-Boot master for almost any
RISC-V board, the messages are in two types, one is about duplicated
symbols,

	u-boot/arch/riscv/cpu//mtrap.S:32: multiple definition of `trap_entry';
	arch/riscv/cpu/mtrap.o: u-boot/arch/riscv/cpu//mtrap.S:32: first defined here

and the other is fixdep's complaint about missing dependency files,

	fixdep: error opening file: arch/riscv/cpu/.mtrap.o.d: No such file or directory
	fixdep: error opening file: arch/riscv/cpu//.start.o.d: No such file or directory

where the latter could only be reproduced when building parallelly.

Both the two types of errors are about files in arch/riscv/cpu, and
there's a suspicious slash character in the reported path. Looking
through RISC-V-specific Makefiles, there's only one place that may
expand to such a path,

	libs-y += arch/riscv/cpu/$(CPU)/

The right hand expands to "arch/riscv/cpu//" if $(CPU) isn't defined at
the time of including. With some debug statement added to
arch/riscv/Makefile, the output proves that arch/riscv/Makefile is
included twice, once with $(CPU) undefined and once defined correctly
according to CONFIG_SYS_CPU.

Futher bisecting shows an extra include statement against
arch/$(SRCARCH)/Makefile is added in earlier bump of Kbuild system. But
the statement is evaluated before config.mk is included and definition
of $(CPU), causing objects in arch/riscv/cpu/ are built and linked twice
(once as "arch/riscv/cpu/*", and once as "arch/riscv/cpu//*"), resulting
in the error.

Let's simply remove the extra include to fix these nasty errors. For
config targets, bumping Kbuild also introduced a new include to
arch/$(SRCARCH)/Makefile, which is removed as well for consistency.

Fixes: 5f520875bdf ("kbuild: Bump the build system to 5.1")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Bryan Brattlof <bb@ti.com>