arm: xea: Move XEA's environment variables from xea.h to xea.env

The default set of environment variables from CFG_EXTRA_ENV_SETTINGS
has been moved to a separate file - board/liebherr/xea/xea.env

Adjustments done:
- fitImage support
- SPI-NOR layout re-organization

Signed-off-by: Lukasz Majewski <lukma@denx.de>