Convert CONFIG_SPL_TARGET to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_TARGET

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 027ac42..dfbda1b 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -1583,6 +1583,15 @@
 	help
 	  Load address of the OpenSBI binary.
 
+config SPL_TARGET
+	string "Addtional build targets for 'make'"
+	default "spl/u-boot-spl.srec" if RCAR_GEN2
+	default "spl/u-boot-spl.scif" if RCAR_GEN3
+	default ""
+	help
+	  On some platforms we need to have 'make' run additional build target
+	  rules. If required on your platform, enter it here, otherwise leave blank.
+
 config TPL
 	bool
 	depends on SUPPORT_TPL