tools: convert makefiles to kbuild style

Before this commit, makefiles under tools/ directory
were implemented with their own way.

This commit refactors them by using "hostprogs-y" variable.

Several C sources have been added to wrap other C sources
to simplify Makefile.
For example, tools/crc32.c includes lib/crc32.c

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/tools/fdt_ro.c b/tools/fdt_ro.c
new file mode 100644
index 0000000..9005fe3
--- /dev/null
+++ b/tools/fdt_ro.c
@@ -0,0 +1 @@
+#include "../lib/libfdt/fdt_ro.c"