esp8266/boards/GENERIC_512K: Add custom minimal _boot.py.

Commit 0abf6f830c removed _boot.py from the
manifest for the GENERIC_512K board because the build does not include a
filesystem.  But the main code expects _boot.py to be there and prints an
error if it's not.  So add a custom _boot.py, which just sets the
gc.threshold().

Signed-off-by: Damien George <damien@micropython.org>

ports/esp8266/boards/GENERIC_512K/_boot.py

@@ -0,0 +1,3 @@
+import gc
+
+gc.threshold((gc.mem_free() + gc.mem_alloc()) // 4)

ports/esp8266/boards/GENERIC_512K/manifest.py

@@ -1,3 +1,4 @@
+freeze("$(BOARD_DIR)", "_boot.py", opt=3)
 freeze("$(PORT_DIR)/modules", ("apa102.py", "neopixel.py", "ntptime.py", "port_diag.py"))
 freeze("$(MPY_DIR)/drivers/dht", "dht.py")
 freeze("$(MPY_DIR)/drivers/onewire")