core: load IPE policy on boot

IPE is a new LSM being introduced in 6.12. Like IMA, it works based on a
policy file that has to be loaded at boot, the earlier the better. So
like IMA, if such a policy is present, load it and activate it.

If there are any .p7b files in /etc/ipe/, load them as policies.
The files have to be inline signed in DER format as per IPE documentation.

For more information on the details of IPE:

https://microsoft.github.io/ipe/

src/basic/build.c

@@ -45,6 +45,12 @@ const char* const systemd_features =
         " -IMA"
 #endif
 
+#if ENABLE_IMA
+        " +IPE"
+#else
+        " -IPE"
+#endif
+
 #if ENABLE_SMACK
         " +SMACK"
 #else