efi_loader: Fall back to fdtfile naming convention

When there is no $fdtfile variable set, we still have a good chance
that on 32bit arm the fdtfile really is just called $soc-$board.dtb.

Enable the exports for $soc and $board in our distr defaults and make
use of them in the efi boot script.

Reported-by: Andreas Faerber <afaerber@suse.de>
Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
diff --git a/include/config_distro_defaults.h b/include/config_distro_defaults.h
index 076be4d..5cc2af8 100644
--- a/include/config_distro_defaults.h
+++ b/include/config_distro_defaults.h
@@ -65,5 +65,6 @@
 #define CONFIG_ISO_PARTITION
 #define CONFIG_SUPPORT_RAW_INITRD
 #define CONFIG_SYS_HUSH_PARSER
+#define CONFIG_ENV_VARS_UBOOT_CONFIG
 
 #endif	/* _CONFIG_CMD_DISTRO_DEFAULTS_H */