main/postmarketos-initramfs: show OS version on splash screen (MR 4470)

This shows the OS version (from /etc/os-release), e.g. "edge", on the splash
screen. os-release is a tiny text file, adding it to the initramfs doesn't seem
too bad, and having this information on the splash screen can be helpful.

Note that this uses "VERSION" and not "PRETTY_NAME" from the os-release file,
since the splash already shows "postmarketOS" (and the pretty name includes
that too), it seemed redundant and took up valuable display space.

[ci:skip-build]: already built successfully in CI