KEYS: Make the system trusted keyring depend on the asymmetric key type

Make the system trusted keyring depend on the asymmetric key type as
there's not a lot of point having it if you can't then load asymmetric keys
onto it.

This requires the ASYMMETRIC_KEY_TYPE to be made a bool, not a tristate, as
the Kconfig language doesn't then correctly force ASYMMETRIC_KEY_TYPE to
'y' rather than 'm' if SYSTEM_TRUSTED_KEYRING is 'y'.

Making SYSTEM_TRUSTED_KEYRING *select* ASYMMETRIC_KEY_TYPE instead doesn't
work as the Kconfig interpreter then wrongly complains about dependency
loops.

Signed-off-by: David Howells <dhowells@redhat.com>

certs/Kconfig

@@ -17,6 +17,7 @@ config MODULE_SIG_KEY
 config SYSTEM_TRUSTED_KEYRING
 	bool "Provide system-wide ring of trusted keys"
 	depends on KEYS
+	depends on ASYMMETRIC_KEY_TYPE
 	help
 	  Provide a system keyring to which trusted keys can be added.  Keys in
 	  the keyring are considered to be trusted.  Keys may be added at will

crypto/asymmetric_keys/Kconfig

@@ -1,5 +1,5 @@
 menuconfig ASYMMETRIC_KEY_TYPE
-	tristate "Asymmetric (public-key cryptographic) key type"
+	bool "Asymmetric (public-key cryptographic) key type"
 	depends on KEYS
 	help
 	  This option provides support for a key type that holds the data for