build/stage1: support local systemd source for offline builds

[dm0-]

Local systemd source trees are useful here for packagers to build usr_from_src without requiring network access. This differentiates between local and remote systemd sources by checking if the value from ./configure contains "://". If the string is found, the build will preserve existing behavior. If not, the value is interpreted as a direct specification of the source directory path.

[lucab]

@fabiokung would you mind a review and perhaps a quick test that it doesn't break your setup?

[fabiokung]

@lucab will do.

Meanwhile, I've been using --with-stage1-systemd-src=file:///path/to/local/clone on a local clone without network access successfully for a while. Is that good enough, or do we really need to support non-git repos locally?

[dm0-]

This is for building RPMs where you'd only have the source archive, so yes, it is best to not require Git.

[fabiokung]

My custom stage1-src flavor build works fine with this. Only two nits:

• --with-stage1-systemd-revision is a noop when the systemd src dir is not a git repo. Add a few sentences explaining this on ./configure --help (or docs elsewhere)?
• make clean will wipe your local systemd directory, which bit me a few times as I was pointing to a local source directory I was working on. Consider disabling the rm -rf ${RKT_STAGE1_SYSTEMD_SRC} when it is a local dir, or copy contents elsewhere for the build.

Otherwise LGTM.

[dm0-]

I've added a note to the revision option description, and I've changed the Makefile to copy the local directory instead of using it directly. This should handle make clean properly, and it's probably better for using non-temporary systemd source paths anyway, since the build system would write additional files into the source directory.

[fabiokung]

cool, re-tested and LGTM.

I should mention that I didn't have any custom patch files being applied to systemd, so I didn't test that. But glancing at the code applying them, it looks fine to me.

[lucab]

LGTM