buildsystem: save/load build config, simplify clean and build --all

[MilhouseVH]

Following on from #2516, this is an attempt to save some "state" within each build directory so that it doesn't have to be parsed from the build directory name which isn't going to be reliable/easy in every case.

Have I missed anything (any variables, reasons why this won't work)?

It's not perfect, I realise it's possible to rebuild with different DEBUG values each time, so the presence of DEBUG is somewhat informational and probably shouldn't be relied upon, but would represent the last value used.

The config is only saved when executing scripts/image - not sure if this is going to be a problem with addons. I can't imagine anyone builds the main image exclusively using scripts/build.

[MilhouseVH]

Just thinking about the scripts/build --all <pkg> change... if you have an le9.0 (ie. master) and an le8.2 build directory then running scripts/build --all <pkg> will attempt to build the package in both build directories (in sequence) using the current branch which will be wrong for at least one of the build directories, resulting in carnage. So use with caution...

Edit: Maybe we could store the current branch, and validate it on load (return 1 if wrong branch)?

[MilhouseVH]

I've added an update which checks that the current branch is the same branch used to create the build.

[HiassofT]

Checking if the branch matches seems counter-productive to me. I use feature/topic/merge branches a lot when testing new features which means the branch name would never match.

I'm using separate worktrees for each LE repository branch (8.2, master, rockchip) so all builddirs within a tree are built - roughly - from the same LE version.

Switching back and forth between eg 8.2 and master in the same tree looks like a rather odd thing to me. If this is an issue we want to solve, it should be handled at major version level, not branch name. Or just state that this is not supported.

[MilhouseVH]

Ok, dropped branch check. Anyone using this will just need to be wary when switching branches.