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>

board/cssi/cmpc885/Makefile

@@ -5,6 +5,6 @@
 # Christophe Leroy <christophe.leroy@c-s.fr>
 #
 
-obj-y += cmpc885.o ../common/common.o
+obj-y += cmpc885.o
 obj-y += sdram.o
 obj-$(CONFIG_CMD_NAND) += nand.o

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

board/cssi/common/Makefile

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright (C) 2024 CS GROUP France
+# Christophe Leroy <christophe.leroy@csgroup.eu>
+#
+
+obj-$(CONFIG_TARGET_CMPC885) += common.o
+obj-$(CONFIG_TARGET_CMPCPRO) += common.o