Allow setting plaform macro settings externally

[zboszor]

Yocto has their own way to set the platform names via BSPs (Board Support Packages). These names are more specific than basic CPU architectures, and such a platform name ends up in /etc/rpm/platform but the corresponding subdirectory under /usr/lib/rpm/platform does not exist.

Allow creating such custom platform subdirectory with feeding the necessary data using external variables: RPM_CUSTOM_ARCH, RPM_CUSTOM_ISANAME, RPM_CUSTOM_ISABITS, RPM_CUSTOM_CANONARCH and RPM_CUSTOM_CANONCOLOR

Fixes #2578

[ffesti]

This PR needs to be against master. Also a feature like this needs at least some documentation. installplatform is a script no one ever should need to look into. So hiding features in there is not acceptable. May be adding a section to INSTALL is a good first step. We can move that to the docs/manual later on if we feel it take up too much space there.

I am also wonder if we should allow passing the directory name. I might have missed something but it looks like there can only be a "custom" platform.

So please re-open this against master.

[zboszor]

This PR needs to be against master. Also a feature like this needs at least some documentation. installplatform is a script no one ever should need to look into. So hiding features in there is not acceptable. May be adding a section to INSTALL is a good first step. We can move that to the docs/manual later on if we feel it take up too much space there.

I added the documentation text in INSTALL after the default make install line.

I am also wonder if we should allow passing the directory name.

I don't know. At this point it's still RFC both for rpm and the Yocto community. But it allows me to run rpmbuild straight on the target system.

I might have missed something but it looks like there can only be a "custom" platform.

One is enough. There is only a single target platform you can build for at a time.

See the complete patch at https://lists.openembedded.org/g/openembedded-core/message/184844

So please re-open this against master.

See #2585