sdcp: Add SDCP base class

This adds a base class for SDCP devices. Not all functionality has been
fully tested, in particular the code to verify the model certificate is
most likely broken or incomplete. One problem there is that there is no
code to find the root CA to trust.

See: #257

meson.build

@@ -217,6 +217,13 @@ foreach i : driver_helpers
             error('nss is required for @0@ and possibly others'.format(driver))
         endif
 
+        optional_deps += nss_dep
+    elif i == 'sdcp'
+        nss_dep = dependency('nss', version: '>=3.55', required: false)
+        if not nss_dep.found()
+            error('nss >=3.55 is required for SDCP support (@0@ and possibly others)'.format(driver))
+        endif
+
         optional_deps += nss_dep
     elif i == 'udev'
         install_udev_rules = true