Remove linux-postmarketos-mainline/stable (or: use linux-lts/custom kernel for N900)

We don't maintain linux-postmarketos-mainline/stable very well. mainline is stuck on 5.4-rc1 (now: 5.6-rc3) and stable is stuck on 5.3.2 (now: 5.5.6). Part of the reason for this may be that it is barely used by anything.

Alpine has linux-lts, which is actively updated with the LTS kernel updates. This should be the ultimate goal for all devices – to have a stable kernel that is actively updated with fixes (especially for security updates). When a device has reached a point where it does no longer need any custom patches it should be ideally moved to such an actively updated (generic) kernel.

IMO there is no need to duplicate that effort in postmarketOS. We should upstream all necessary options and then use the Alpine kernels directly.

We already have some devices using Alpine kernels:

• qemu-amd64, qemu-aarch64: linux-lts or linux-virt
• tablet-x64uefi: linux-lts
• surftab-wintron7.0: linux-lts
• raspberry-*: linux-rpi

At the moment, the only devices using linux-postmarketos-mainline/stable are:

• qemu-amd64, qemu-aarch64: addressed in !981 (closed)
• qemu-vexpress: completely broken at the moment, @Minecrell will try to fix it (probably by attempting to make it more similar to qemu-aarch64)
• sony-amani: probably better suited with linux-postmarketos-qcom-msm8974
• nokia-n900

The only device that really needs linux-postmarketos-stable at the moment is the nokia-n900. Someone with the device should investigate using linux-lts. We might run into one of the following problems:

• The are config options missing in linux-lts: make a MR upstream
• The kernel is too large for nokia-n900: The same thing would have happened with linux-postmarketos-stable at some point. In that case, we need a custom kernel or at least a custom kernel configuration. (e.g. linux-lts upstream already builds multiple kernel configurations...) (should not be a problem because kernel is on SD card...)

The general approach for using generic kernels is to add necessary modules to deviceinfo_modules_initfs.