Kbuild: introduce Makefile in arch/$ARCH/

Introduce a Makefile under arch/$ARCH/ and include it in the
top Makefile (similar to Linux kernel). This allows further
refactoringi like moving architecture-specific code out of global
makefiles, deprecating config variables (CPU, CPUDIR, SOC) or
deprecating arch/$ARCH/config.mk.

In contrary to Linux kernel, U-Boot defines the ARCH variable by
Kconfig, thus the arch Makefile can only included conditionally
after the top config.mk.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/sh/Makefile b/arch/sh/Makefile
new file mode 100644
index 0000000..ca55fac
--- /dev/null
+++ b/arch/sh/Makefile
@@ -0,0 +1,8 @@
+#
+# SPDX-License-Identifier:	GPL-2.0+
+#
+
+head-y := arch/sh/cpu/$(CPU)/start.o
+
+libs-y += arch/sh/cpu/$(CPU)/
+libs-y += arch/sh/lib/