powerpc/mpc85xx: use board env file for socrates board

as Tom suggested get rid of CFG_EXTRA_ENV_SETTINGS and
enable CONFIG_ENV_SOURCE_FILE and use text file

board/socrates/socrates.env

which contains the default environment. While at it,
cleanup the default Environment.

Signed-off-by: Heiko Schocher <hs@denx.de>
Suggested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>