env: Enable getenv_f() for SPL_BUILD

With this patch, getenv_f() can be included easily into the SPL
binary. With this, SPL boards can now use getenv_f() to read
environment variables (e.g. to detect if the OS or U-Boot shall
be executed).

In the approach this is done for env stored in NOR flash, as this
will be used by an upcoming MPC5200 board port.

Signed-off-by: Stefan Roese <sr@denx.de>