mx6sxsabresd: Convert to the new Kconfig style

mx6sxsabresd was not in the master branch when the conversion to the new Kconfig
style happened, so convert it now so that it can build again.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/configs/mx6sxsabresd_defconfig b/configs/mx6sxsabresd_defconfig
new file mode 100644
index 0000000..f23d48f
--- /dev/null
+++ b/configs/mx6sxsabresd_defconfig
@@ -0,0 +1,3 @@
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sxsabresd/imximage.cfg,MX6SX"
+CONFIG_ARM=y
+CONFIG_TARGET_MX6SXSABRESD=y