powerpc/corenet: fix compile error when CONFIG_SYS_NO_FLASH is defined

ENV location compile logic is wrong, and when CONFIG_SYS_NO_FLASH is defined
and non-NOR u-boot is building, it will cause compile error. Also, add
CONFIG_SYS_FLASH_USE_BUFFER_WRITE for p2041, which will improve NOR flash
write performance.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>