Move existing use of CONFIG_SPL_RSA to Kconfig

A few boards define this in a header file which is incorrect. It means that
Kconfig options that rely on this cannot be used. Move it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/T1040QDS_SECURE_BOOT_defconfig b/configs/T1040QDS_SECURE_BOOT_defconfig
index 1b4f654..ae6aacf 100644
--- a/configs/T1040QDS_SECURE_BOOT_defconfig
+++ b/configs/T1040QDS_SECURE_BOOT_defconfig
@@ -31,4 +31,5 @@
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_RSA=y
+CONFIG_SPL_RSA=y
 CONFIG_OF_LIBFDT=y