autoboot.c: Fill env vars in process_fdt_options() only if TEXT_BASE is set

The x86 build target "efi-x86" has no TEXT_BASE configured. And with the
introduction of CONFIG_BOOTDELAY for x86, this function is now called
for this board as well. Resulting in compile errors for this target.

Without TEXT_BASE it makes no sense to fill these values. So lets only
configure the env variable if TEXT_BASE is defined.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>