Release 0.4.0

Bootloader size

   text	   data	    bss	    dec	    hex	filename
  17396	      8	     20	  17424	   4410	bootloader

Firmware size

   text	   data	    bss	    dec	    hex	filename
 159132	   1512	   8196	 168840	  29388	air-gradient-pro