env: Access Environment in SPI flashes before relocation

Enable the new Kconfig option ENV_SPI_EARLY if you want
to use Environment in SPI flash before relocation.
Call env_init() and than you can use env_get_f() for
accessing Environment variables.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>