Convert CONFIG_SYS_BOOT_RAMDISK_HIGH to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOT_RAMDISK_HIGH

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/boot/Kconfig b/boot/Kconfig
index e7a4d53..17438b5 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -606,6 +606,14 @@
 	  Enables addition of RCW (Power on reset configuration) in built image.
 	  Please refer doc/README.pblimage for more details.
 
+config SYS_BOOT_RAMDISK_HIGH
+	depends on CMD_BOOTM || CMD_BOOTI || CMD_BOOTZ
+	depends on !(NIOS2 || SANDBOX || SH || XTENSA)
+	def_bool y
+	help
+	  Enable initrd_high functionality.  If defined then the initrd_high
+	  feature is enabled and the boot* ramdisk subcommand is enabled.
+
 endmenu		# Boot images
 
 menu "Boot timing"