env: Add a bootfile env handler

Remove the hard-coded bootfile handler and use a callback instead

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

include/env_callback.h

@@ -39,6 +39,7 @@
  * a new association in the ".callbacks" environment variable.
  */
 #define ENV_CALLBACK_LIST_STATIC ENV_CALLBACK_VAR ":callbacks," \
+	"bootfile:bootfile," \
 	CONFIG_ENV_CALLBACK_LIST_STATIC
 
 struct env_clbk_tbl {