On Low disk space try to upload readings. Plus sleep on boot

When low disk space detected try to upload the readings to free up the disk space.  On Boot sleep for 0.5 seconds at start.

main.py

@@ -23,10 +23,15 @@
 # from phew import logging
 # logging.disable_logging_types(logging.LOG_DEBUG)
 
+# Issue #117 where neeed to sleep on startup otherwis emight not boot
+from time import sleep
+sleep(0.5)
+
 # import enviro firmware, this will trigger provisioning if needed
 import enviro
 import os
 
+
 try:
   # initialise enviro
   enviro.startup()
@@ -42,7 +47,19 @@
   if enviro.low_disk_space():
     # less than 10% of diskspace left, this probably means cached results
     # are not getting uploaded so warn the user and halt with an error
-    enviro.halt("! low disk space")
+
+    # Issue #126 to try and upload if disk space is low
+    # is an upload destination set?
+    if enviro.config.destination:
+      enviro.logging.error("! low disk space. Attempting to upload file(s)")
+
+      # if we have enough cached uploads...
+      enviro.logging.info(f"> {enviro.cached_upload_count()} cache file(s) need uploading")
+      if not enviro.upload_readings():
+        enviro.halt("! reading upload failed")
+    else:
+      # no destination so go to sleep
+      enviro.halt("! low disk space")
 
   # TODO this seems to be useful to keep around?
   filesystem_stats = os.statvfs(".")