mx6sabreauto: Update to SPL only mode

As mx6sabreauto supports SPL now, all variants can boot using the same
defconfig.

This patch:
- Removes non-SPL targets.
- Renames target to mx6sabreauto_defconfig.
- Renames folder and board files to mx6sabreauto.
- Updates MAINTAINERS, Makefile and Kconfig accordingly.
- Removes .cfg files.
- Adds a README with instructions to build and flash SPL and u-boot.img.

Signed-off-by: Vanessa Maegima <vanessa.maegima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
diff --git a/board/freescale/mx6sabreauto/Makefile b/board/freescale/mx6sabreauto/Makefile
new file mode 100644
index 0000000..87f4ec0
--- /dev/null
+++ b/board/freescale/mx6sabreauto/Makefile
@@ -0,0 +1,9 @@
+#
+# Copyright (C) 2007, Guennadi Liakhovetski <lg@denx.de>
+#
+# (C) Copyright 2011 Freescale Semiconductor, Inc.
+#
+# SPDX-License-Identifier:	GPL-2.0+
+#
+
+obj-y  := mx6sabreauto.o