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/configs/mx6sabreauto_defconfig b/configs/mx6sabreauto_defconfig
new file mode 100644
index 0000000..c1b3bbf
--- /dev/null
+++ b/configs/mx6sabreauto_defconfig
@@ -0,0 +1,53 @@
+CONFIG_ARM=y
+CONFIG_ARCH_MX6=y
+CONFIG_SPL_GPIO_SUPPORT=y
+CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_TARGET_MX6SABREAUTO=y
+CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_LIBDISK_SUPPORT=y
+CONFIG_SPL_WATCHDOG_SUPPORT=y
+CONFIG_VIDEO=y
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,SPL,MX6QDL"
+CONFIG_BOOTDELAY=3
+# CONFIG_CONSOLE_MUX is not set
+CONFIG_SYS_CONSOLE_IS_IN_ENV=y
+CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
+CONFIG_SPL=y
+CONFIG_SPL_EXT_SUPPORT=y
+CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_HUSH_PARSER=y
+CONFIG_CMD_BOOTZ=y
+# CONFIG_CMD_IMLS is not set
+# CONFIG_CMD_FLASH is not set
+CONFIG_CMD_MMC=y
+CONFIG_CMD_SF=y
+CONFIG_CMD_I2C=y
+CONFIG_CMD_USB=y
+CONFIG_CMD_DFU=y
+CONFIG_CMD_GPIO=y
+CONFIG_CMD_DHCP=y
+CONFIG_CMD_MII=y
+CONFIG_CMD_PING=y
+CONFIG_CMD_CACHE=y
+CONFIG_CMD_EXT2=y
+CONFIG_CMD_EXT4=y
+CONFIG_CMD_EXT4_WRITE=y
+CONFIG_CMD_FAT=y
+CONFIG_CMD_FS_GENERIC=y
+CONFIG_DFU_MMC=y
+CONFIG_DFU_SF=y
+CONFIG_MTD_NOR_FLASH=y
+CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_USB=y
+CONFIG_USB_STORAGE=y
+CONFIG_USB_GADGET=y
+CONFIG_CI_UDC=y
+CONFIG_USB_GADGET_DOWNLOAD=y
+CONFIG_G_DNL_MANUFACTURER="FSL"
+CONFIG_G_DNL_VENDOR_NUM=0x0525
+CONFIG_G_DNL_PRODUCT_NUM=0xa4a5
+# CONFIG_VIDEO_SW_CURSOR is not set
+CONFIG_OF_LIBFDT=y