define prowl partition layout

fixes for quantenna file storage in u-boot env:
  - u-boot env capped at 24K
  - offset to quantenna files should start at 24K, not 4K

fixes for default environment
  - wire up SRAM based default environment
  - allow saving of default environment to flash
  - environment changes before first saveenv+reset are persisted

bake default gfrg240 board configuration into code, so that a
board with no environment comes up and is able to configure the

Change-Id: I24eaa2dc5512286a79488b41087d4d0a799786d7
8 files changed