mvebu: enable CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE=y config option

mvebu: enable CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE=y config option

Enable the CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE kernel option to allow for
U-Boot environment writing. This might be hiding a problem somewhere else,
since the w25q128fw chip supports 32K erases, still this change makes it
much easier to switch the GL-MV1000 boot media without an UART cable
connection.

Thanks to @robimarko and @hacks for the precious hints and suggesting a
better approach.

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>