Document changes and release 5.12.2pureos2 of linux-librem5

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
2 jobs for release5122_2 in 66 minutes and 32 seconds
Status Job ID Name Coverage
  Build
passed #349753
librem5
build:aarch64

00:38:28

passed #349754
librem5:arm64
build:deb

01:06:32