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: da595d236b ("include: configs: soc64: Use CONFIG_SPL_ATF to differentiate bootfile")
Signed-off-by: Tom Rini <trini@konsulko.com>

env/Kconfig

@@ -957,6 +957,7 @@ config USE_BOOTFILE
 
 config BOOTFILE
 	string "'bootfile' environment variable value"
+	default kernel.itb if SPL_ATF && TARGET_SOCFPGA_SOC64
 	depends on USE_BOOTFILE
 	help
 	  The value to set the "bootfile" variable to.