Add support for empty init arguments in the command line parser

[mschlumpp]

Summary of the PR

For automated scripts it is easier to keep the trailing -- separator than to separately check whether it is strictly necessary.
Without this change the crate would include the trailing -- as a kernel argument. Downstream consumers such as firecracker then proceed to append further parameters to that string (mmio parameters). The final re-assembled string looked similar to this: <kernel params> -- <mmio-args>, therefore passing the mmio parameters not to the kernel but to the init part.

Requirements

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• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[roypat]

Hi @mschlumpp,
Thank you for your PR! I think what you're trying to achieve is already partially supported by linux-loader (e.g. the first of the two test cases you add passes even without your changes). The only missing piece is that if the string passed to try_from has the form <boot args> -- (e.g. no trailing space after the --), then the -- will be considered part of the boot args instead of being stripped. For this, I think we can achieve what you want with a simpler change:

diff --git a/src/cmdline/mod.rs b/src/cmdline/mod.rs
index 2ac7c31..5687ca3 100644
--- a/src/cmdline/mod.rs
+++ b/src/cmdline/mod.rs
@@ -487,6 +487,10 @@ impl Cmdline {
             ),
         };
 
+        if boot_args.ends_with(INIT_ARGS_SEPARATOR.trim_end()) {
+            boot_args = &boot_args[..boot_args.len() - INIT_ARGS_SEPARATOR.trim().len()];
+        }
+
         boot_args = boot_args.trim();
         init_args = init_args.trim();

Given that we already trim the trailing -- if its followed by a space, that seems like a reasonable change to make, but I'd like to get some input from someone who knows more about the intricacies here (maybe @rbradford?).

Other than that, we should also adjust the documentation of try_from, to point out that it strips trailing --s. It should also get a changelog entry.