spl: Ensure all SPL symbols in Kconfig have some SPL dependency

Tighten up symbol dependencies in a number of places.  Ensure that a SPL
specific option has at least a direct dependency on SPL.  In places
where it's clear that we depend on something more specific, use that
dependency instead.  This means in a very small number of places we can
drop redundant dependencies.

Reported-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/lib/rsa/Kconfig b/lib/rsa/Kconfig
index b773f17..9033384 100644
--- a/lib/rsa/Kconfig
+++ b/lib/rsa/Kconfig
@@ -18,6 +18,7 @@
 
 config SPL_RSA
 	bool "Use RSA Library within SPL"
+	depends on SPL
 
 config SPL_RSA_VERIFY
 	bool