Reorganize options targeting mcu

[dNechita]

PR Description

Resolves #1256

PR Type

• [ ] Bug fix (a change that fixes an issue)
• [x] New feature (a change that adds new functionality)
• [ ] Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• [x] I have conducted a self-review of my own code changes
• [ ] I have commented new code, particulary complex or unclear areas
• [ ] I have checked that I did not intoduced new warnings or errors (CI output)
• [ ] I have checked that components that use libiio did not get broken
• [ ] I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[dNechita]

I think this PR has reach its final state in terms of changes which include:

• cmake will turn ON or OFF the relevant options when configuring for Tinyiiod. It's enough to run: cmake -DWITH_LIBTINYIIOD=ON ..
• Once you turn WITH_LIBTINYIIOD to ON and want to turn it OFF again, you need to empty your build dir
• Updates to the check_README_BUILD.sh due to the changes regarding options in CMakeFiles.txt
• Fixes for when building with Tinyiiod configuration
• Documentation updates

While working at this I came across the presets functionality that modern CMake has. I gave it a try and here you can see how it looks like: https://github.com/analogdevicesinc/libiio/compare/main..tinyiiod-preset-options This can be an alternative but:

• we would then need to keep track of the options both in the CMakeFile.txt and CMakePresets.json
• users could still overwrite the options values in the presets file. In the current state of the PR this won't happen.

[rgetz]

cmake presets are only available in cmake > 3.19 (which would eliminate these older distributions).

Operating System	Cmake Version
LinuxBuilds fedora28	cmake version: 3.14.4
LinuxBuilds ubuntu_20_04	cmake version: 3.16.3
ARMBuilds ubuntu-ppc64le	cmake version: 3.16.3
ARMBuilds ubuntu-x390x	cmake version: 3.16.3
ARMBuilds ubuntu-arm64v8	cmake version: 3.16.3
LinuxBuilds debian_bullseye	cmake version: 3.18.4

and leave us with only:

Operating System	Cmake Version
LinuxBuilds opensuse_15_4	cmake version: 3.20.4
LinuxBuilds fedora34	cmake version: 3.20.5
LinuxBuilds ubuntu_22_04	cmake version: 3.22.1
ARMBuilds ubuntu-arm32v7	cmake version: 3.23.2
ARMBuilds debian_bookworm	cmake version: 3.23.2
LinuxBuilds centos_7	cmake version: 3.23.2
LinuxBuilds debian_bookworm	cmake version: 3.25.1
macOSBuilds macOS_13_arm64	cmake version: 3.27.3
WindowsBuilds VS2022	cmake version: 3.31.6
WindowsBuilds VS2019	cmake version: 3.31.6
macOSBuilds macOS_13_x64	cmake version: 3.31.6
MinGWBuilds MinGW_W64	cmake version: 4.0.2

I don't know if that is what you want to do or not?

[dNechita]

I don't know if that is what you want to do or not?

I wouldn't want to lose those older distributions. Additionally, the current implementation works properly without any major drawbacks. Therefore, I would go ahead and merge these changes. In the long term, I can see a transition to cmake presets.