board: cssi: Use HAVE_VENDOR_COMMON_LIB logic

Instead of cross using cross-directory makefile directives,
add a Makefile in board/cssi/common/ directory in order to
benefit from HAVE_VENDOR_COMMON_LIB logic.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
diff --git a/board/cssi/cmpcpro/Makefile b/board/cssi/cmpcpro/Makefile
index 73ff451..3083778 100644
--- a/board/cssi/cmpcpro/Makefile
+++ b/board/cssi/cmpcpro/Makefile
@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y += cmpcpro.o nand.o ../common/common.o
+obj-y += cmpcpro.o nand.o