arm: socfpga: Correct how we set BOOTFILE

arm: socfpga: Correct how we set BOOTFILE

In order to set the BOOTFILE symbol we first need to have USE_BOOTFILE
be set, or some of the logic might not work as expected later on when
building. Second, defaults like this belong with the symbol itself.

Fixes: da595d236b97 ("include: configs: soc64: Use CONFIG_SPL_ATF to differentiate bootfile")
Signed-off-by: Tom Rini <trini@konsulko.com>