kbuild, tools: generate wrapper C sources automatically by Makefile

There are many source files shared between U-boot image and tools.
Instead of adding a lot of dummy wrapper files that just include
the corresponding file in lib/ or common/ directory,
Makefile should automatically generate them.

The original inspiration for this came from
scripts/Makefile.asm-generic of Linux Kernel.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/.gitignore b/tools/.gitignore
index 725db90..0eb9068 100644
--- a/tools/.gitignore
+++ b/tools/.gitignore
@@ -21,3 +21,6 @@
 /easylogo/easylogo
 /gdb/gdbcont
 /gdb/gdbsend
+
+/lib/
+/common/