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 d92cc4d516 ("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)

arch/alpha/Makefile

@@ -55,9 +55,6 @@ $(boot)/vmlinux.gz: vmlinux
 bootimage bootpfile bootpzfile: vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 
-archclean:
-	$(Q)$(MAKE) $(clean)=$(boot)
-
 archheaders:
 	$(Q)$(MAKE) $(build)=arch/alpha/kernel/syscalls all