samsung-manta: move to community

This MR proposes to move Samsung Manta from testing to community.

• Installation instructions must be well documented on device wiki page

Current instructions work OK AFAICT. But the part on the device not using fastboot is not right. I'm waiting for a Wiki account to fix that.

• Must run a (close to) mainline kernel

Running linux-postmarketos-exynos5.

• Kernel must pass pmbootstrap kconfig check --community, with includes working firewall (#1119 (closed))

linux-postmarketos-exynos5 does.

• Must have automatic kernel upgrade working

See 9c12b1bc.

• Maintainer(s) must take part in the workflow for new postmarketOS releases
• Track record of upgrading the kernel, device kernel or SoC kernel must at least have been upgraded through 3 kernel releases

linux-postmarketos-exynos5, being in community already, certainly does.

Outstanding missing features include graphic acceleration and audio. But I am confident these will available as we progress. Indeed:

• Graphic acceleration used to work with an older version of Mesa. In the meantime, I entertain a fork of wlroots to support rendering with the exynos 2D accelerator (see https://github.com/alexmrqt/wlroots/tree/g2d-0.16.2, not mature but feel free to reach if you want to test/improve).
  • NOTE: it has very good chances to get GPU working on next mesa upgrade
• Sound is based on WM1811. It is supported in the mainline kernel (Arndale uses it). But custom board code may have to be written.

Cc @jenneron @rumpel