ARM: 8969/1: decompressor: simplify libfdt builds

Copying source files during the build time may not end up with
as clean code as expected.

lib/fdt*.c simply wrap scripts/dtc/libfdt/fdt*.c, and it works
nicely. Let's follow this approach for the arm decompressor, too.

Add four wrappers, arch/arm/boot/compressed/fdt*.c and remove
the Makefile messes. Another nice thing is we no longer need to
maintain the own libfdt_env.h because the decompressor can include
<linux/libfdt_env.h>.

There is a subtle problem when generated files are turned into
check-in files.

When you are doing a rebuild of an existing object tree with O=
option, there exists stale "shipped" copies that the old Makefile
implementation created. The build system ends up with compiling the
stale generated files because Make searches for prerequisites in the
current directory, i.e. $(objtree) first, and then the directory
listed in VPATH, i.e. $(srctree).

To mend this issue, I added the following code:

  ifdef building_out_of_srctree
  $(shell rm -f $(addprefix $(obj)/, fdt_rw.c fdt_ro.c fdt_wip.c fdt.c))
  endif

This will need to stay for a while because "git bisect" crossing this
commit, otherwise, would result in a build error.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/boot/compressed/fdt.c

@@ -0,0 +1,2 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include "../../../../lib/fdt.c"