botan: Use strongSwan's RNG interface in Botan plugin

This allows using rng_t implementations provided by other plugins to
serve as RNG for Botan.

Closes strongswan/strongswan#192.

conf/Makefile.am

@@ -33,6 +33,7 @@ plugins = \
 	plugins/attr.opt \
 	plugins/attr-sql.opt \
 	plugins/bliss.opt \
+	plugins/botan.opt \
 	plugins/bypass-lan.opt \
 	plugins/certexpire.opt \
 	plugins/coupling.opt \

conf/plugins/botan.opt

@@ -0,0 +1,6 @@
+charon.plugins.botan.internal_rng_only = no
+	Force the use of Botan's internal RNG.
+
+	If enabled, only Botan's internal RNG will be used throughout the plugin.
+	Otherwise, and if supported by Botan, rng_t implementations provided by
+	other loaded plugins will be used as RNG.