board/seco: Add mx6q-uq7 basic board support

Add basic SECO MX6Q/uQ7 board support (Ethernet, UART, SD are supported).
It also adds a Kconfig skeleton to later add more SECO board (supporting
SoC and board variants).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
diff --git a/board/seco/common/Makefile b/board/seco/common/Makefile
new file mode 100644
index 0000000..9e37ebb
--- /dev/null
+++ b/board/seco/common/Makefile
@@ -0,0 +1,2 @@
+
+obj-$(CONFIG_TARGET_SECOMX6) += mx6.o