vexpress_ca9x4: Enable use of correct DTB file and restore EFI loader.

EFI was disabled in f95b8a4b5f64f because of the missing DTB file,
and indeed, the DTB file is required to load recent versions of GRUB
(2.04) correctly.

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>
diff --git a/configs/vexpress_ca9x4_defconfig b/configs/vexpress_ca9x4_defconfig
index 2119df6..6bd1f25 100644
--- a/configs/vexpress_ca9x4_defconfig
+++ b/configs/vexpress_ca9x4_defconfig
@@ -34,4 +34,4 @@
 CONFIG_BAUDRATE=38400
 CONFIG_CONS_INDEX=0
 CONFIG_OF_LIBFDT=y
-# CONFIG_EFI_LOADER is not set
+CONFIG_DEFAULT_FDT_FILE="vexpress-v2p-ca9.dtb"