esp8266/modules/flashbdev: Change RESERVED_SECS to 0.

This effectively reverts the change that introduced this new constant.
The reason is so that users do not need to rebuild the filesystem on
their modules when upgrading the firmware.

Users can change RESERVED_SECS by hand if they need the feature, and in
future firmware it may default to a non-zero value.

esp8266/modules/flashbdev.py

@@ -3,7 +3,7 @@ import esp
 class FlashBdev:
 
     SEC_SIZE = 4096
-    RESERVED_SECS = 1
+    RESERVED_SECS = 0
     START_SEC = esp.flash_user_start() // SEC_SIZE + RESERVED_SECS
     NUM_BLK = 0x6b - RESERVED_SECS