mx6sl: Select MX6SL option via Kconfig

Currently the MX6SL option is selected via CONFIG_SYS_EXTRA_OPTIONS,
but it is better to select it directly via Kconfig.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/mx6/Kconfig b/arch/arm/mach-imx/mx6/Kconfig
index 44d2db4..2b6bc1b 100644
--- a/arch/arm/mach-imx/mx6/Kconfig
+++ b/arch/arm/mach-imx/mx6/Kconfig
@@ -244,6 +244,7 @@
 
 config TARGET_MX6SLEVK
 	bool "mx6slevk"
+	select MX6SL
 	select SUPPORT_SPL
 
 config TARGET_MX6SLLEVK
@@ -396,6 +397,7 @@
 
 config TARGET_WARP
 	bool "WaRP"
+	select MX6SL
 	select BOARD_LATE_INIT
 
 config TARGET_XPRESS