Fix the bug that devtool modify u-boot is not picked up by image-bmc

Previously when u-boot code was changed inside a devtool modify repo, the
u-boot artifacts were updated but they could not be picked up by the
final image-bmc. The reason was that the sstate version of u-boot was
used because the dependency between u-boot and obmc-phosphor-image was
not set up properly.

Fix the issue by adding the proper inter-task dependency between the
two.

Tested:
The image-bmc md5sum changes as expected when u-boot code is changed.

Change-Id: Ic98d3097f66e351c774ced4ee435149b5441cf67
Signed-off-by: Yong Zhao <yozhao@google.com>
diff --git a/meta-phosphor/classes/image_types_phosphor_nuvoton.bbclass b/meta-phosphor/classes/image_types_phosphor_nuvoton.bbclass
index 0909273..8fefd82 100644
--- a/meta-phosphor/classes/image_types_phosphor_nuvoton.bbclass
+++ b/meta-phosphor/classes/image_types_phosphor_nuvoton.bbclass
@@ -22,6 +22,7 @@
 }
 
 do_prepare_bootloaders[depends] += " \
+    u-boot:do_deploy \
     npcm7xx-bootblock:do_deploy \
     npcm7xx-bingo-native:do_populate_sysroot \
     npcm7xx-igps-native:do_populate_sysroot \