[sw/lib] Modify existing documentation comments to use doxygen format.

Also moved documentation of some interfaces from .c to .h files.
diff --git a/sw/lib/flash_ctrl.h b/sw/lib/flash_ctrl.h
index 544afcc..d0ba0d9 100644
--- a/sw/lib/flash_ctrl.h
+++ b/sw/lib/flash_ctrl.h
@@ -41,7 +41,7 @@
  */
 int flash_check_empty(void);
 
-/*
+/**
  * Erase flash bank |bank_idx|. Blocks until erase is complete.
  *
  * @param idx Flash bank index.