esp8266/_boot.py: Decrease GC alloc threshold to quarter of heap size.

The idea behind decrease is: bytecode and other static data is also kept on
heap, and can easily become half of heap, then setting threshold to half of
heap will have null effect - GC will happen on complete heap exhaustion like
before. But exactly in such config maintaining heap defragmented is very
important, so lower threshold to accommodate that.

esp8266/modules/_boot.py

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