TT#45401 lib/get_debian_release: support Debian/buster + Debian/bullseye

Currently `/etc/debian_version` for Debian/buster systems includes:

buster/sid

So until we get the according version number via base-files, also
check for the codename instead. Make it more generic so it works for
any upcoming release that uses ".*/sid", to not run into the issue
again with e.g. the upcoming Debian/release codenamed bullseye (the
release expected after buster).

Thanks: Manuel Montecelo for improving my initial version

Change-Id: I59e6adea98b6c228daa2b5ebe14872c4a35b8920

lib/get_debian_release

@@ -19,6 +19,8 @@
     $debian_codename = 'buster';
   } elsif ($debian_version =~ /^11\.*/) {
     $debian_codename = 'bullseye';
+  } elsif ($debian_version =~ /^[[:alnum:]]+\.*\/sid/) {
+    $debian_codename = (split /\//, $debian_version, 2)[0];
   }
   die "Fatal error retrieving Debian release codename [$debian_codename]" unless length $debian_codename;
   die "Fatal error retrieving Debian release version [$debian_version]" unless length $debian_version;