pylibfdt: compile pylibfdt only when dtoc/binman is necessary

Currently, pylibfdt is always compiled if swig is installed on your
machine.  It is really annoying because most of targets (excepts
x86, sunxi, rockchip) do not use dtoc or binman.

"checkbinman" and "checkdtoc" are wrong.  It is odd that the final
build stage checks if we have built necessary tools.  If your platform
depends on dtoc/binman, you must be able to build pylibfdt.  If swig
is not installed, it should fail immediately.

I added PYLIBFDT, DTOC, BINMAN entries to Kconfig.  They should be
property select:ed by platforms that need them.  Kbuild will descend
into scripts/dtc/pylibfdt/ only when CONFIG_PYLIBFDT is enabled.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>