mx7: Place MX7_SEC option in Kconfig

MX7_SEC is an existing configuration option that allows booting the
kernel in secure mode.

Place this option in Kconfig, so that boards can select this option
in their defconfig files.

Selecting this option is necessary when booting a kernel provided by
NXP, such as 3.14_GA and 4.1.15_GA.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Michael Trimarchi <michael@amarulasolutions.com>

arch/arm/imx-common/Kconfig

@@ -17,3 +17,12 @@ config IMX_BOOTAUX
 	depends on ARCH_MX7 || ARCH_MX6
 	help
 	  bootaux [addr] to boot auxiliary core.
+
+config MX7_SEC
+	bool "Support booting MX7 in secure mode"
+	depends on ARCH_MX7
+	help
+	  NXP kernel requires to boot MX7 in secure mode. Select
+	  this options if you want to boot a NXP kernel. In order
+	  to boot a mainline kernel this option needs to be
+	  unselected.