resources: Update x86-ubuntu build script

The current script src/x86-ubuntu/disk-image/build.sh looks for
the packer script at boot-exit/boot-exit.json. This change updates
the script to look for the packer script at x86-ubuntu/x86-ubuntu.json
instead (relative path from build.sh).

Change-Id: I17111477dc3743d0b306c5cf1c10ae97fca71840
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/65312
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/src/x86-ubuntu/disk-image/build.sh b/src/x86-ubuntu/disk-image/build.sh
index e5e4978..935eaa6 100755
--- a/src/x86-ubuntu/disk-image/build.sh
+++ b/src/x86-ubuntu/disk-image/build.sh
@@ -6,5 +6,5 @@
     rm packer_${PACKER_VERSION}_linux_amd64.zip;
 fi
 
-./packer validate boot-exit/boot-exit.json
-./packer build boot-exit/boot-exit.json
+./packer validate x86-ubuntu/x86-ubuntu.json
+./packer build x86-ubuntu/x86-ubuntu.json