Allow Python packages to be dropped

When building in a portage chroot, we do not have the environment needed
to build pylibfdt. It is instead build as a separate package.

Provide a build option to tell U-Boot to skip this part of the build. We
still need it to use binman, etc. but don't need it to build its
dependencies.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
[s/build bytes/builds bytes in tools.rst]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

scripts/dtc/Makefile

@@ -19,4 +19,6 @@ HOSTCFLAGS_dtc-parser.tab.o := -I$(src)
 $(obj)/dtc-lexer.lex.o: $(obj)/dtc-parser.tab.h
 
 # Added for U-Boot
+ifeq ($(PYTHON_ENABLE),y)
 subdir-$(CONFIG_PYLIBFDT) += pylibfdt
+endif