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/include/configs/mx6qsabreauto.h b/include/configs/mx6sabreauto.h
similarity index 94%
rename from include/configs/mx6qsabreauto.h
rename to include/configs/mx6sabreauto.h
index 99b48ca..fd0d5ce 100644
--- a/include/configs/mx6qsabreauto.h
+++ b/include/configs/mx6sabreauto.h
@@ -6,8 +6,8 @@
  * SPDX-License-Identifier:	GPL-2.0+
  */
 
-#ifndef __MX6QSABREAUTO_CONFIG_H
-#define __MX6QSABREAUTO_CONFIG_H
+#ifndef __MX6SABREAUTO_CONFIG_H
+#define __MX6SABREAUTO_CONFIG_H
 
 #ifdef CONFIG_SPL
 #include "imx6_spl.h"
@@ -76,4 +76,4 @@
 #define CONFIG_POWER_PFUZE100
 #define CONFIG_POWER_PFUZE100_I2C_ADDR	0x08
 
-#endif                         /* __MX6QSABREAUTO_CONFIG_H */
+#endif                         /* __MX6SABREAUTO_CONFIG_H */