esp8266/_boot.py: Set GC alloc threshold to half of heap size.

Should keep good chunk of heap unfragmented, if a user application allows
that at all.

esp8266/modules/_boot.py

@@ -1,4 +1,5 @@
 import gc
+gc.threshold((gc.mem_free() + gc.mem_alloc()) // 2)
 import uos
 from flashbdev import bdev