uVisor: Enforce thread switch atomicity

The system hook thread_switch gets called both through the SVC and
though the PendSV, which can get interrupted by the SVC.
Since calls to the uVisor API are not re-entrant, we need to wrap this
function in an atomic section to prevent faults.

features/FEATURE_UVISOR/AUTHORS.txt

@@ -1,7 +1,7 @@
-   568	Milosch Meriac
-   477	Alessandro Angelino
-    72	Jaeden Amero
-    53	Niklas Hauser
+   569	Milosch Meriac
+   478	Alessandro Angelino
+    73	Jaeden Amero
+    54	Niklas Hauser
      3	Hugo Vincent
      3	JaredCJR
      3	Jim Huang

features/FEATURE_UVISOR/VERSION.txt

@@ -1 +1 @@
-v0.25.0
+v0.25.1