kbuild: use more subdir- for visiting subdirectories while cleaning

Documentation/kbuild/makefiles.rst suggests to use "archclean" for
cleaning arch/$(SRCARCH)/boot/, but it is not a hard requirement.

Since commit d92cc4d51643 ("kbuild: require all architectures to have
arch/$(SRCARCH)/Kbuild"), we can use the "subdir- += boot" trick for
all architectures. This can take advantage of the parallel option (-j)
for "make clean".

I also cleaned up the comments in arch/$(SRCARCH)/Makefile. The "archdep"
target no longer exists.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 0eb4568..753e13f 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -1,7 +1,5 @@
 # This file is included by the global makefile so that you can add your own
-# architecture-specific flags and dependencies. Remember to do have actions
-# for "archclean" and "archdep" for cleaning up and making dependencies for
-# this architecture
+# architecture-specific flags and dependencies.
 #
 # This file is subject to the terms and conditions of the GNU General Public
 # License.  See the file "COPYING" in the main directory of this archive
@@ -137,6 +135,3 @@
 install zinstall:
 	$(CONFIG_SHELL) $(srctree)/$(boot)/install.sh $(KERNELRELEASE) \
 	$(boot)/$(install-image) System.map "$(INSTALL_PATH)"
-
-archclean:
-	$(Q)$(MAKE) $(clean)=$(boot)