ath79: improve support for Letv LBA-047-CH

ath79: improve support for Letv LBA-047-CH

1. Convert wireless calibration data to NVMEM.
2. Enable control green status LED and change default LED behaviors.
   The three LEDs of LBA-047-CH are in the same position, and the green
   LED will be completely covered by the other two LEDs. So don's use
   green LED as WAN indicator to ensure that only one LED is on at a time.
   LED     Factory          OpenWrt
   blue    internet fail    failsafe && upgrade
   green   internet okay    run
   red     boot             boot
3. Reduce the SPI clock to 30 MHz because the ath79 target does not
   support 50 MHz SPI operation well. Keep the fast-read support to
   ensure the spi-mem feature (b3f9842330) is enabled.
4. Remove unused package "uboot-envtools".
5. Split the factory image into two parts: rootfs and kernel.
   This change can reduce the factory image size and allow users to
   upgrade the OpenWrt kernel loader uImage (OKLI) independently.

   The new installation method: First, rename "squashfs-kernel.bin" to
   "openwrt-ar71xx-generic-ap147-16M-kernel.bin" and rename "rootfs.bin"
   to "openwrt-ar71xx-generic-ap147-16M-rootfs-squashfs.bin". Then we
   can press reset button for about 5 seconds to enter tftp download mode.
   Finally, set IP address to 192.168.67.100 and upload the above two
   parts via tftp server.

Tested on Letv LBA-047-CH

Signed-off-by: Shiji Yang <yangshiji66@qq.com>