sandbox: CONFIG_SYS_RELOC_GD_ENV_ADDR?

[AKASHI Takahiro]

Tom, Simon,

Is CONFIG_SYS_RELOC_GD_ENV_ADDR really needed on sandbox?

When I try to have a variable environment on emulated SPI flash,
the U-Boot binary always crashes: (NOTE: assuming CONFIG_ENV_ADDR == 0)
    $ dd if=/dev/zero of=./spi.bin bs=1M count=4
    $ u-boot -T
    U-Boot 2020.04-rc2-00015-gc9afef2b1938-dirty (Feb 14 2020 - 10:24:59 +0900)

    Model: sandbox
    DRAM:  128 MiB
    WDT:   Started with servicing (60s timeout)
    MMC:   mmc2: 2 (SD), mmc1: 1 (SD), mmc0: 0 (SD)
    Loading Environment from SPI Flash... SF: Detected m25p16 with page size 256 Bytes, erase size 64 KiB, total 2 MiB
    *** Warning - bad CRC, using default environment

    Segmentation fault (core dumped)

If this configuration is disabled, panic doesn't happen.
I think that it should be turned off in any sandbox*_defconfig.

In addition, please update
- doc/arch/sandbox.rst
- doc/SPI/README.sandbox-spi
Both two still mention already-removed command line option, --spi_sf.
It is confusing.

Thanks,
-Takahiro Akashi