Fix: define SIZE_2_8_ROUND_USB constant

  Commit a3a375d (PR #995) referenced SIZE_2_8_ROUND_USB in five places
  in configure.py but never defined the constant, causing a NameError at
  import time. This adds the missing definition.

Author: wyldere

configure.py

@@ -78,6 +78,7 @@
 SIZE_8_8_INCH_NEWREV = "8.8\" / 9.2\" (V1.X new HW rev.)"
 SIZE_12_3_INCH = "12.3\""
 SIZE_2_8_INCH_NEWREV = "2.8\" round (V1.X new HW rev.)"
+SIZE_2_8_ROUND_USB = "2.8\" round (USB)"
 
 # List of sizes that can be selected
 size_list = (