modsign: Add explicit CONFIG_SYSTEM_TRUSTED_KEYS option

Let the user explicitly provide a file containing trusted keys, instead of
just automatically finding files matching *.x509 in the build tree and
trusting whatever we find. This really ought to be an *explicit*
configuration, and the build rules for dealing with the files were
fairly painful too.

Fix applied from James Morris that removes an '=' from a macro definition
in kernel/Makefile as this is a feature that only exists from GNU make 3.82
onwards.

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

init/Kconfig

@@ -1752,6 +1752,19 @@ config SYSTEM_TRUSTED_KEYRING
 
 	  Keys in this keyring are used by module signature checking.
 
+config SYSTEM_TRUSTED_KEYS
+	string "Additional X.509 keys for default system keyring"
+	depends on SYSTEM_TRUSTED_KEYRING
+	help
+	  If set, this option should be the filename of a PEM-formatted file
+	  containing trusted X.509 certificates to be included in the default
+	  system keyring. Any certificate used for module signing is implicitly
+	  also trusted.
+
+	  NOTE: If you previously provided keys for the system keyring in the
+	  form of DER-encoded *.x509 files in the top-level build directory,
+	  those are no longer used. You will need to set this option instead.
+
 config SYSTEM_DATA_VERIFICATION
 	def_bool n
 	select SYSTEM_TRUSTED_KEYRING