gitlab-ci: Switch to Ubuntu 18.04 image

In order to run all filesystem tests we need to have newer ext4 tools,
move up to Ubuntu 18.04 'bionic' for our base.  We need to change
slightly how we invoke the provided grub-mkimage.  This will also make
future python3 work easier.
Signed-off-by: NTom Rini <trini@konsulko.com>

.gitlab-ci.yml

@@ -2,7 +2,7 @@
 
 # Grab our configured image.  The source for this is found at:
 # https://gitlab.denx.de/u-boot/gitlab-ci-runner
-image: trini/u-boot-gitlab-ci-runner:xenial-20190720-02Aug2019
+image: trini/u-boot-gitlab-ci-runner:bionic-20190912.1-03Oct2019
 
 # We run some tests in different order, to catch some failures quicker.
 stages:
@@ -22,8 +22,9 @@ stages:
     - . /tmp/venv/bin/activate
     - pip install pytest==2.8.7
     - pip install python-subunit
-    - grub-mkimage -o ~/grub_x86.efi -O i386-efi normal  echo lsefimmap lsefi lsefisystab efinet tftp minicmd
-    - grub-mkimage -o ~/grub_x64.efi -O x86_64-efi normal  echo lsefimmap lsefi lsefisystab efinet tftp minicmd
+    - pip install coverage
+    - grub-mkimage --prefix="" -o ~/grub_x86.efi -O i386-efi normal  echo lsefimmap lsefi lsefisystab efinet tftp minicmd
+    - grub-mkimage --prefix="" -o ~/grub_x64.efi -O x86_64-efi normal  echo lsefimmap lsefi lsefisystab efinet tftp minicmd
     - mkdir ~/grub2-arm
     - ( cd ~/grub2-arm; wget -O - http://download.opensuse.org/ports/armv7hl/distribution/leap/42.2/repo/oss/suse/armv7hl/grub2-arm-efi-2.02~beta2-87.1.armv7hl.rpm | rpm2cpio | cpio -di )
     - mkdir ~/grub2-arm64