powerpc/corenet_ds: Correct the compilation errors about ENV

When defined CONFIG_ENV_IS_NOWHERE, there will be some
compilation errors:

./common/env_nowhere.o: In function `env_relocate_spec':
./common/env_nowhere.c:38: multiple definition of `env_relocate_spec'
./common/env_flash.o: ./common/env_flash.c:326: first defined here
./common/env_nowhere.o: In function `env_get_char_spec':
./common/env_nowhere.c:42: multiple definition of `env_get_char_spec'
./common/env_flash.o:./common/env_flash.c:78: first defined here
./common/env_nowhere.o: In function `env_init':
./common/env_nowhere.c:51: multiple definition of `env_init'
./common/env_flash.o:./common/env_flash.c:237: first defined here
make[1]: *** [./common/libcommon.o] Error 1
make[1]: Leaving directory `./common'
make: *** [./common/libcommon.o] Error 2

Remove the CONFIG_ENV_IS_IN_FLASH if defined CONFIG_ENV_IS_NOWHERE.

Signed-off-by: Liu Gang <Gang.Liu@freescale.com>