Merge lp:~ltrager/maas-images/fix_building_bootloaders into lp:maas-images

[Lee Trager]

Fix building bootloaders.

This fixes bootloaders failing to build due to being unable to verify the
deb file GPG key and works around LP:1790003 by specifying a prefix when
using grub-mkimage.

[Scott Moser (smoser)]

As long as gpgv takes multiple --keyring arguments as you imply here the change looks fine.

Just some comments about readability. I've just learned to do subprocess code like this over time to (possibly) avoid having to type 'man some-command' in order to figure out:

  some-command -acf foo --flag1 --flag2 foo2 bar

Just better to do:
  some-command --activate --configure --file=foo --flag1 --flag2=foo2 bar

=== modified file 'meph2/commands/dpkg.py'
--- meph2/commands/dpkg.py	2018-06-25 19:15:30 +0000
+++ meph2/commands/dpkg.py	2018-08-30 21:46:10 +0000
@@ -46,8 +46,13 @@
         stream.write(data_file)
 
     subprocess.check_output(
-        ['gpgv', '--keyring', '/etc/apt/trusted.gpg', sig_out, data_out],
-        stderr=subprocess.STDOUT)
+        [
+            'gpgv',
+            '--keyring', os.path.join(
+                os.path.dirname(__file__), '..', '..', 'keyring.gpg'),
+            '--keyring', '/usr/share/keyrings/ubuntu-archive-keyring.gpg',
+            sig_out, data_out
+        ], stderr=subprocess.STDOUT)
 
     shutil.rmtree(tmp, ignore_errors=True)
 
@@ -310,6 +315,7 @@
                  '-O', grub_format,
                  '-d', modules_path,
                  '-c', grub_config_path,
+                 '-p', '',
                  ] + modules)
         else:
             subprocess.check_output(